Keeping a percula with a clarkii clown
 

I currently have a 10 g tank with a clarkii clowfish and a BTA that he is hosting. I was wondering if it would be a problem to add a very small percula clownfish to the system?

Huge problem. Don't mix clowns in small tanks. My percula is now in my fuge because the Clarkii almost killed it and I have a 55 gallon tank.

What do you recommend to keep with a clarkii? I had a clown goby at one point for a few months, but he vanished.

The perc will be dead in no time. There's really not much else you could put in a 10 gallon where a clarkii has already established territory other than trying another small goby perhaps.