Avoid harlequins in a 5 gallon - they are quite boisterous, are schooling fish and do better in a 20 gallon.
I have a nerite snail with my betta in a 10 gallon, and it does great. ;)
10 Gallon Tank:
~Countless Pond Snails~1 Espe's Rasbora~1 Harlequin Rasbora~
5 and 5.5 Gallon Tanks:
Avatar by Fenghuang
Many of my scaly friends from my original tank from 2011 have passed, except for two! I am in the process of re-doing my tank. :)