The citron gobie and neon gobies will be fine with the blenny.......the algae blenny shouldn't be kept with watchman type gobies as they are the same body style and tend to compete in space......citron gobies are great little perching fish who stay up in the rockwork or perch on the glass.....at least mine does and he is home with a wheeler watchman gobie....and they have never fought in a 20g.
I question two grammas as I have read if they are not a mated pair will not co habitate....