(don't forget about the filter!)
For the 5g tank you could get some of these micro-species;
celestrial danio (3/4")
pygmy corydoras (3/4")
ember tetra (3/4")
scarlet badis (3/4")
guppy
endlers
betta
shrimp
I probably missed afew. Most of these are softwater fish. They have all been aproved by experts. Most don't get more than an inch.