Can I ask which country your in? Its just that the US and UK have different measurements for gallons, and this slight difference can be quite big when it comes to actual tank volume/ room.
Tank size/ volume wise I always feel that the biggest you can go is best, so I would advise using the 75G as your main tank.
If the swordtails are all male along with the platy then they can be mixed, otherwise they will interbreed.
The guppies will be fine with the swordtails and platy as long as there are no females swordtails or the platy, since the male guppies can make real pests of themselves trying to mate with any females whether they are a female guppy or not.
Provided you don't add aquarium salts/ or tonics to increase the water hardness (even salts for medicating) to the tank then the BN plec, coryodras and khuli loaches will be fine with the other fish, but keep in mind most guppies, platies, and swordtails come from hard water, while BN's, corys, and khuli loaches come from soft water habits along with gouramis. Ideally some more corys will help your current 4 be less stressed and usually more active about the tank. But wait till your ich issues are sorted first.
The gourami may find the usually zippy guppies and danios a bit bewildering and off putting, is it a dwarf species or one of the larger species of gourami? Because gouramis are often more sedate and measured when feeding make sure the faster fish don't get all the food and the slower gourami misses out. The same problem can occur for the corydoras and khuli loaches that hang around the bottom of the tank. To counteract the greedy fast mouths you can do night feeds for the BN, corys and khuli loaches which actually suits their habits better.
