i am using tanks such as 6 fters, and a couple of 4 fters to breed my discus...more water less changes required
have seen some posts saying a 2 ft tank is fine but my fish are sizeable enough that they wouldn't be able to swim/turn around in this sized tank so the best thing i can say is to look at the size of the fish and decide if they will be happy or stressed in the size tank you want to put them into.
as for adding other fish later, if you are separating the pairs via egg crate or some other form that will allow the fry to swim through you might find the other pair (if not with fry themselves) eating them. if you put in a solid partition so you had (in effect) two tanks then you might have more luck keeping fry
as for when to move the fry i usually wait two weeks after they are feeding off the parents sides so as to be sure they are hardy enough to withstand the move and will be able to eat baby brine and other high protein foods.
others on this forum may have different practices (and btw if anyone is breeding in 2 fters can you let me know the size of the fish please)