Any fish you add are purely for your enjoyment which is a great reason to add fish. Your tank can have more fish. Consider a Orchid Dottyback pair, or a Royal Gramma trio, or a hawk fish, or a Mandarin , or shrimp goby with shrimp. There are a lot of smaller reefish for small tank like your.
Consider a flame angel or one of the smaller angel in that tank.
Any fish that you added will not benifit your clowns, but will not cause problem for the clowns either.
For my 65 g tank I have the following fish:
A. percula (Onyx) female, pair with a A. ocellaris male(Snowflake)
Flame angel
Two Yellow tail blue damsels
a Atlantic blue tang.
The tang is getting large, pushing 4.5 inches. I will need to remove him and put him into my 300 g tank. I got him when he was quarter size. All my fish have been in my tank for at least 5 years (the clowns) The other have been there for 8+ years (I had another pair of clowns before that in there) The damsel are getting old but still spawn every few weeks. The clown spawn every 9 days.
Attached is a picture of this tank