IMHO you are close to max. 25G. The damsel and clowns are all territorial and get big eventually.
If you are set on one more - royal gramma, some kind of small gobie, or a firefish are some of the popular ones.
Alternatively if you just want more "creatures" in the tank, may I suggest either tux urchin, cleaner, pistol, peppermint or coralband shrimp, hermit crabs etc.
Pistol shrimp can also pair up with a gobie for an interesting symbiosis. They are bottom dwelling as well so may have less territory conflicts with the clown/damsel.