I do not recommend keeping bettas with peaceful fish. There are some circumstances where it works (like in your case), but the more you add, the more you may run into compatibility issues.

I strongly advise to not get the knife fish with your current fish stock or adding another betta. Bettas are solitary fish.

Angelfish are usually picked on by faster swimming fish, and the gouramis are semi- or completely aggressive depending on the species.

I know it seems like I'm shutting your plan down, but I just can't advise to keep different temperaments of fish together. It may work for you, and it may be that the knife fish eats every last fish in your tank.

From what I read, knife fish should be solitary. I don't know from personal experience, however, researching is there to assist your decisions. I can tell you've researched.

Also, be careful adding too many fish at once because you can offset the balance of beneficial bacteria in the tank, which will lead to ammonia poisoning in your fish.

Hope this helps.

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