Have you considered micro fish for your 20g long? Not sure how they will fair with the GBRs but I had Celestail Perl Danio in my grow out tank with GBRs.
I actually put 10 Burmese Zebra Rasboras in my 20g long this week after relocating the BN pleco breeding trio that was in there. (not much breeding going on these days...LOL) I need to nurse them back to health due to sunk stomachs but they took to the live food quick.