Of course compatibility matters.
I tend to go with a non mathematical approach. Depends on size of fish, bio load, your filtration, etc. if unsure, it's best to come up with a list and get feedback on it. Don't ask a LFS
as they will likely give questionable advice.
The OP was asking about putting a Niger Trigger in a 55 that already had a few fish. Compatibility with a Trigger is always tricky. Some go crazy and start killing tankmates after being a model resident for a long time. I would not risk it in a smaller tank just for that reason.