newland, this friend of yours... how long has he been keeping these guppies & platies? until a tank hits 6 months, its not mature. its really best to not consider any fishkeeping enterprise a sucess until you've had no problems for about a year.
a 5g could have a betta and not much else. dwarf platies (if you can find them) only get about 1.5 inches long, so 4 of those could work... that's my latest acquisition, so i've been recommending them to everyone.

tres cute. or you could get 3 2inch tetras. schools tend to disentegrate when no threats are present, so 3 of a tetra wouldn't be stressful provided
that's the only kind of fish you have in there
i've kept a SAP in a heavily planted 5g for a year, with one companion to help eat his scraps (a guppy fry that outgrew puffer's appetite, lately a khulii loach). a dwarf puffer would do much better in a 5g and would have a lot more personality than a betta. it would also be much more likely to slaughter any tankmates, but...

you might could keep a male/female pair of dp, provided the 5g isn't a hex and is well-planted with plenty of hidey-holes.
a 10g would let you have a betta and a small school of something like cory cats or a smaller tetra. or you could have a betta girl colony (4-6) provided the tank is very heavily planted. a 10g would provide much more flexibility than a 5g, but you'd still be limited on what you could put in it.
a 15g would be the best of the small tanks. you could have a couple of nice schools or one school and a bigger fish. several people on here have kept a pair of German Rams in a 15g with neons. a sensitive mix, but gorgeous. pretty much any of the dwarf cichlids could be kept and bred in a 15g with no problems.