Size really depends on which species you are planning to get.
Insignis- will reach 16''....
Taenerus- will reach about 8'', you can comfartably fit about 5-7 in there.
About the damage to scaleless fish, I have yet to see my guys do that to any fish. I have had polys, and niger cats in with them. I feed them plenty of veggies, so there is no need of doing damage.....
Thanks, the geo jurupari is good sized now atleast 9-10''