Best fish for my community tank?

[MorganCarr]

I have a twenty gallon community tank. I cycled it then put in a male betta, 2 cory cats, and 3 platys, each new species put in a month apart. They have all got along so far and are fairly active. My tank has black gravel and plenty of silk plants because i'm kinda afraid to put in live plants and plastic plants look like they arent very comfortable if a fish wanted to rest in/on them. I have two small caves and the temp stays 75-78 degrees F. I want to add a small shoal (5-6) of fish next without risking my bettas fins getting nipped. I have two fish stores nearby, petland and a LFS who told me they get their fish the same place as petland. Between the two these are my choices: Glofish and Zebra Danios, cherry barbs, guppies (i know they arent a shoaling fish but theyre cute), Glowlight tetra, Neon tetra, harlequin rasbore, gold barbs and rosy barbs. Also, I saw a jack dempsey with cotton mouth at petsmart today but he is in a hospital tank. Is it still safe to buy there?