Put the pads and the ceramic media into the new filter. Much of the bacteria will be on the little ceramic noodles in the sealed box, and you don't want to lose them. After a few weeks you can gradually remove the media the PF2 had from the Eheim, and replace it with new Eheim stuff. Or just leave it in there.
Guppies in a 70L will be fine, but as you're keeping all males I'd plant well. The livebearers I keep can get pretty feisty with each other, and as you won't have females to distract them with, plants that will break up lines of sight, and allow them to hide from each other will help.
Re: Hi, I'm new and just about to set up my first tank
As the space won't allow anything much over 100L or so I wouldn't worry too much about the weight, its not as if you're trying to fit a 600L monster up there.
If you're buying new, I'd definitely look at the Juwel 800 if I were in your position. The Fluval equivalent (Roma 90), and anything bigger from either manufacturer will be rather more expensive.
Going secondhand there's a huge range of choice. Clearseal tanks are good, but if you buy them new all you'll get is a tank, and you'll have to get the filters, hood, lights, heater, stand and foam mat separately, which will probably work out at about what the Juwel kit does, but will be more awkward to put together when you're just starting out. They're definitely worth looking at on the second-hand market however, and the 30"x12"x15" would fit in the space you have.
Depending on his temperament you may be able to house him with some pygmy corydoras- Corydoras habrosus would probably be the best choice as it spends more of its time at the bottom than C. pygmaeus or C. hastatus, the other commonly seen dwarf cories. You'll need 6 for a shoal, and a sand substrate.
The Eheim Scubacubes are nice, but they don't come with a filter, and being ehiem aren't cheap. I think I'd look at the AquaOne AquaCube, as its rather more affordable.
They may be OK with harder water than most other cories, but even they won't like a pH of 8.
What type of test kit is it? Strips or liquid? Strips are much less accurate than the liquid kits, so you may be lucky and be seeing a pH rather higher than you actually have. If you can reduce your pH and hardness on this tank too you may be OK, but you're likely to be making compromises between what the guppies and cories need, so neither may be particularly happy. You'd be better off looking at some of the smaller Rift Lake Catfish instead of cories if you want to keep livebearers.
You won't need to do a fishless cycle on the new tank. Take your existing medai and squidge it into the new filter when you transfer the fish. Same amount of bacteria, same amount of fish, bigger volume of tank, and you'll be able to add lots more media to the filter for the colony to grow. Rinse and repeat for the 60L. Depending on the filter you could probably just swap them over.
First the bad news, even 450L won't be big enough for a Silver Shark. They get to nearly a foot long, are very active, skittish fish, and need to be in shoals (6+). You'll need something more like 700L and 6'+ for them. I recommend rehoming them ASAP. They don't mix well with plants either.
Corydoras won't like the hard alkaline water that you have I'm afraid, and I can't think of many bottom-dwellers that will, and will also like the tropical conditions that the current stock needs.
Best thing to do with the tetras will be to reduce the pH gradually mixing RO and tapwater over a series of small water changes, or use all RO and remineralising salts to get the desired pH and hardness. Are you using test strips or liquid dropper kits? You'll need the liquid kits as they're much more accurate for measuring your KH and GH (alternatively invest in a TDS meter, which are relatively inexpensive) and you'll need to test every bucket of water you make up.
In terms of water changes you'll need to do 25-30% waterchanges per week on either tank. On my 240L that works out at 5-6 14L buckets, so you're basically going to have to do 10-12 buckets- 150L or so a week. You can either ferry lots of buckets around, or use a bit of kit called a "python".
ND Aquatics do either a 36"x24"x24" or a 36"x24"x18", which come in at around ?400-?425. They're very nice looking tanks, but they don't come with any equipment as standard, so you'll need to buy a filter, lights, etc, total comes in at around ?630 though, which isn't too bad for a new tank- you can spend much more and get rather less. (If I had the space I'd love one of their 60x24x24's).