Is that the pH after the water's been left to stand for 24 hours?
If so your water would suit gouramis, tetras, loaches (there are several suitable for smaller tanks- which one you get would depend on the size of your tank), corydoras catfish, lots of "plecs", rasboras, and South American Cichlids. It will not be suited to most livebearers (apart from a couple of unusual, and rarely seen ones), the AFFs, or Rift Lake Cichlids (although these aren't fish you would keep in a general community anyway).
So you have lots of choice there, and a whole range of potential communities to build. I'm a big fan of biotopes, so you could have a nice Amazon-themed tank with tetras and Corydoras catfish, and a small "plec", or a Asian one with Loaches, barbs/rasboras and gouramis.
I would recommend having one shoal of mid-upper water swimmers (danios/rasboras/barbs/tetras), one of bottom dwellers (loaches/corydoras), and a centrepiece fish (gourami trio/cichlid pair). I think one larger shoal looks much nicer than two smaller shoals.
If ammonia and nitrite are zero after 24 hours, but not 12 then you're nearly done. You just have to wait a bit for the population to grow big enough. Each cycle is different, but you'll probably be done in a couple of weeks.
Yes, both ammonia and nitrite need to be at zero after 12 hours.
Generally you'll only need to do water changes if the cycle stalls. As part of the metabolic pathway that the bacteria use to process ammonia and nitrite they also use carbonate dissolved in the water. If your water is very soft they can run out and the cycle stalls. People tend to notice that either ammonia or nitrite isn't breaking down, and nitrates are very high (most people don't test for carbonate unless they're adjusting their water). A big water change, and redosing with ammonia usually solves this problem. If your water is hard you're not likely to have to worry.
The only other time to do water changes when fishless cycling is if your tank has fully cycled, but you can't get to the shops very soon. Keep it ticking over and treat it like you would a normally stocked fish tank.
I'm considering getting a 30x12x12 Clearseal tank and glass sliding covers, and using it with an Arcadia Ellipse I already have. However my question is how deep in the tank are the supports for the cover slides, and will this physically fit? Its been mentioned before, but if anyone can confirm this I'd be grateful.
Siamese Fighters are solitary and generally not community fish. They're aggressive and do not mix with gouramis- or indeed any fish that they view as being either similar to themselves (long tails) or compete for similar parts of the tank (gouramis).
Tetras all need to be in shoals of 6 or more, and generally the bigger the better. Rasboras and danios are the same. Corydoras are also shoaling fish and need sand as a substrate or their barbels get abraded leaving them open to potentially fatal infections.
Gouramis, Clown Loaches, Corydoras, Kribensis, rasboras, the particular danios and tetras all like soft acidic water and temperatures around 24-26 degrees. SFFs also like it soft and acidic, but rather warmer.
Clown Loaches get far to big for your tank. They are also shoaling fish and get to a foot long or so. You'll need a tank about 6' long or more for them. Depending on the "plec" they get very big too- Common Plecs grow to 18" or so, but there are lots of different species and exactly how large they grow, and what food and other conditions they require depends on the species.
Goldring Butterfly Suckers are a type of Hillstream loach. These need unheated tanks with lots of flow and a supply of algae covered pebbles to feed them. They aren't compatible with the other fish you've chosen for this reason (Gouramis and SFFs, as well as being tropical like tanks with lots of vegetation and relatively little current).
American Flag Fish are aggressive, fin-nipping bullies, especially the males. They like densely planted tanks, hard, alkaline water, and need to be kept with fish that are either robust or fast enough to avoid attention. I wouldn't keep them in tanks with SFFs and Gouramis even if they did like the same conditions. A 240L tank would be ideal for a male and a group of females (if planted densely you could probably have more than one male). However they also don't need heated tanks (although they can be kept in tropical tanks).
A good rule of thumb for stocking is 1cm/2L which would give you space for about 120cm of stock, although you can go over this if your maintenance is good. Once you know what your water parameters are you can narrow down your stock somewhat, but the best advice I can give is to choose a species or two that you like the most and build your tank and community around their needs.
Checker Barbs, Cherry Barbs, and Odessa Barbs all get to about 5cm (Odessas can be up to 7cm), so are idea candidates for the 1cm/2L (1"/gallon) "rule", and a 100L tank gives you space for about 50cm of stock, so a shoal of 8-10 would be fine.
Rosy Barbs are rather larger, Wikipedia says up to 14cm, Seriouslyfish about 9-10cm at most, but more usualy 7.5cm (3"), so I'd not keep more than 6 in a 100L.
The plants you have will be absolutely fine if you buy a heater and turn it on (you'll need a 100W for a 100L tank). Hornwort doesn't grow roots at all, but the Anacharis will do. Personally I'd take them out of the pots as it will restrict their growth.
Cherry Barbs will need a heater, but the Odessas, Checkers, and Rosys will be fine without one as long as your tank stays above 20 degrees (which shouldn't be a problem in a centrally-heated home).
In a 100L tank a shoal of 8-10 Cherry Barbs, Checker Barbs, or Odessa Barbs would be ideal. They're roughly the same colour as Goldfish, don't get bigger than a couple of inches, and like the same sort of water conditions (hard and alkaline). They'd be perfect. They're also not nearly so destructive on plants, so you could have some nice easy plants like Vallis, Bacopa, etc, that aren't entirely sacrificial.
You should be able to turn the flow down on the filter, the blue lever on the front should control the flow. (I've not used that particular filter, but most internals are pretty similar).
However, I have some bad news for you. Your fish will need a much bigger tank. Common Goldfish (which you have, and which won't mind the flow that much as they're pretty strong swimmers compared to Fancy Goldfish) grow to about a foot long. You will need a 300L+ tank for the three you have, but a pond would be better. They're small, so they should be OK in the current tank for a little while longer, but I'm afraid another upgrade will be necessary in the near future.
Zebra Danios would be fine in hard alkaline water, as would Leopard Danios (a variety with spots rather than stripes). Its the species that like more tropical temperatures that won't like it as hard and alkaline as you have it.
Guppies don't need to be in shoals, but they are social, and will do better if there's more of them.
I don't know if Cherry and Odessa Barbs would shoal together, I've not kept either, although I'd like to set up a tank for Odessas if I had the space. I would work on the basis that they won't just in case, and choose one or other species, and buy tankmates that will suit their preferred temperature range.