The thing is, that you can keep any kind of clowns in really any size tank over 30 gallons, however, to try and keep two different kinds in a tank less than 6ft is a real gamble.
What I would try to do, is remove the clarkii temporarily, to another tank, without its BTA (making sure the cinnamons don't decide to host it). Then after about a week and a half, you should be able to put him/her back, once the Cinnamons have established a terriory of their own. Then put the clarkii back.
You should be fine. LMK if you have any other questions.