The biggest problem is that fish are individual and don't always act/react as their species would suggest they should. That being said, it is best to follow the knowledge gained over the years and posted on specific web sites.
I like
Drs. Foster and Smith for basic fish info and their
compatibility chart. So, a goby is fine, but two that look alike are probably not.
Firefish are jumpers, so get a net for the aquarium top. BRS sells make your own top netting.
I have a bi-color dotty and it hides from the tangs most of the time. I expected it to be more aggressive and hold it's own, but that only happens at feeding time. Otherwise it never bothers my other (larger) fish. The royal gramma is also a good choice, but it's a Caribbean fish and I wanted an Indo-Pacific species tank. I don't know if that matters to you or not.
I agree about the conflicting info on the Kaudern's Cardinals. I initially wanted a group, but they were unavailable at the time so I got a pair of spotted (pajama) cardinalfish.
Hoping someone here has the Kaudern's and can give you their experience with them.
So much depends on the tank size too. With a 6' long 125 I can offer territory and hiding space to allow me to keep some smaller, but incompatible fish. In your 40 I would heed the warnings. If you have some reefers nearby, you can check in advance if someone will take a problem fish off your hands, so you don't have to experience the fight to the death that sometimes happens.