Ok, so the corys will roughly reach 3inches each at adult size, apart from the leopard who will reach about 2.5inches. Total so far 14.5inches of fish.
You have a 15 gallon - so if we allow 2" of fish per gallon (old guide is 1" per gallon but with good filtration and weekly water changes/maintenance you can up it to 2"), that leaves you with about another 15inches or so of further fish.
Do you like shoaling fish?
I'm thinking that you could have a shoal of tetras - lots to choose from and they don't grow big - neons are about 1" and colour up beautifully if tank lights are left on for min 8 hrs per day. I have silver-tip tetras and they look great in a shoal (a kind of lemony colour with eye-catching silver tips to their tail fins that appear to glow under the light). All of these tetras look great against a dark background or dark gravel.
Or there are beautiful sailfin mollies - pearly white ones, dalmation, black, orange... You could have a pair of sailfins along with a small shoal (6 - 8) of tetras...
Platys are easily available and very easy to keep - they come in all sorts of colour variations.
There are dwarf cichlids you could have in a 2ft tank but they are not necessarily the easiest of fish to keep and they can be overly-sensitive to water conditions and stress easily.
Dwarf gouramis are also quite colourful but, again, not the easiest to keep as they are prone to ulcers and other illnesses.
Anyway, I'm sure you'll get more ideas from others on here... main thing is when making your list is to look for compatibility and don't overstock your tank.
Regards - Athena