I think that should work. Firefish are very hardy, as long as they have a little "bolt hole"/home in the rocks.
I do not have cardinal experience, but I have done a lot of research since I've been thinking of getting some, but they should be fine. Cardinalfish are only aggressive with each other.
There's a small chance the wrasse will pick on the firefish, but not for sure. Once firefish get well established, and have a little territory they actually can hold their own a bit (in my experience keeping a purple one).
All Dartfish are pretty cool in my opinion.