Why do you think you can't keep a cleaner shrimp in a 20 gallon tank? I'm kinda confused. I just added a fire shrimp (which are bigger than cleaner shrimp) to my 3 gallon pico tank.
I'm just scared the fish will be intimated by the shrimp and any future fish I may get. Maybe I'm in a mindset of getting a coral banded shrimp which are just scary imo.
The fish you get will be fine with them, neon gobies are tiny tiny fish and mine does just fine with the shrimp. I don't even think they ever pay attention to each other.