Mollies aren't really compatible as they need hard water where the angelfish and otos are soft water fish. I would go for a shoal of one of the larger bodied tetras (tank size permitting) such as the Buenos Ares Tetra or Colombian Tetra, To minimise territorial behaviour from your angel you can try taking him/her out of the tank, rearrange the decor and put the new fish in before putting him back.
I strongly suggest you get a bottle of ammonia to do your fishless cycle as the fish food method is very unreliable. You need to be able to accurately dose 3 ppm or so in one go, and top back up to that level when ammonia drops, which you can't do with fish food as it takes time (and is resistant to) break down. You can use the ammonia calculator at the end of our fishless cycling article to work out he right doses. https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm
Plants that attach to wood and rocks are the easiest, but you can plant some in gravel if you pop a root tab under them. Java moss, Java fern and Anubias species are good for attaching to wood. Vallis and Amazon swords are fairly easy to grow in the gravel with a root tab. You can add plants at any stage.
Goldfish definitely are social creatures so do better with another goldfish companion. You can mix different types of fancy, but due to the way they have been bred to get the shapes they have, they are all more or less disabled in their swimming ability or eyesight. For that reason it's best to mix similarly compromised types rather than say a moor and a fantail as the fantail would be more boisterous and better at finding food than the moor, who may end up getting little food. I would go with two moors in at least a 180 litre tank and go bigger for more than two.
It depends on what substrate is used - planting substrates can release ammonia and cause an immediate bacterial bloom., or can be stirred up when adding water and cause cloudiness. If it's a milky colour it sounds like a bacterial bloom which will resolve over a few days.
If the tank was left for 8 weeks the beneficial bacteria in the filter would have died back to just enough to support the snails. A lot more would be needed to support the guppies so I suspect ammonia/nitrite have spiked. What test kit are you using? Is it liquid based or is it paper strips? The snails are very unlikely to be the cause of the problem. Another possibility is that the water is too soft for guppies. They are a hard water fish and need the gH to be between 150-500 and preferably in the middle of that range. If you are using test strips though, your readings for all parameters including gH may not be accurate. Modern farmed guppies are also genetically very weak, and in many cases arrive carrying bacteria which has been suppressed when they were bred and grown on by use of antibiotics in the water. Once they arrive in someones tank, these bacteria can take hold, especially if parameters are off for the fish as well.
Do you have test results for ammonia, nitrite and nitrate? I am suspicious that this has been caused by new tank syndrome and iif you stilll have ammonia and/or nitrite those need addressing. Other than thta a good combined anti-bacteria/anti-fungal medication is needed. This should be effective: https://www.amazon.co.uk/Blagdon-Anti- ... id=pla-421768565448&psc=1
I think it would have risen just the same if you hadn't changed water - it was on it's way up so that continued . Honestly, your fish are at risk from the nitrite and water changes are the only way to get it down. The only other possibility is that you have nitrite in your supply - have you tested it? Definitely use a double dose of Prime as that will neutralise the nitrite and allow your fish to breathe properly. Nitrite affects the blood and prevents it from carrying oxygen efficiently, which is what kills fish.