Next fish for nano tank?

[Grizzly_Jordy]

Hi, y'all i have a 20 gallon nano reef tank with a clown fish, tailspot blenny, a purple firefish, and a skunk cleaner shrimp.
My CUC are about 4 dwarf hermit crabs and about a total of 8 various snails.
I used to have a female clown fish that was basically paired to the remaining clown i have but she got too aggressive to the blenny and firefish so i had to trade her in.
Would it be safe to add another clown back and see if they could pair again i don't want more aggression to happen?
Or what other fish can i add to this tank?
My ideas were a banggai cardinal, pink streaked wrasse, maybe a pistol goby pair but I'm worried about the tank size and hurting my cleaner shrimp.

 

[mann1139]

Clown could be a bully.

Wrasse is even more likely to bully.

Cardinal is a good idea if there is an overhang for it to claim.

Blenny and gobies might be a risk.

 

[Reeflix]

any cardinal should do.