pygmy cories or aspidoras are real tiny. there are some really small rasboras too. for a 10g i vote for harlequin. there are alot of tetras that would be ok in a 10g too, along with ur typical snails, shrimp, dwarf frogs ect. And for the 5g a betta of course in that list
I vote darters, if youre gonna have them in alone, very interesting fish active, and responsive, liitle perching hawks. but youll need to add a small powerhead to give plenty water movement at the bottom