Welcome aboard!!!
I like to do fish first actually. In a new tank, there isn’t much for the CUC to eat nor is there really a need for them, other than aesthetics. If you choose your CUC for an actual purpose, it’s easier to choose the correct kind. Once you add a fish or two, you start feeding the tank and then you get nutrients and you’ll see algae growth and who knows what else. You can then add CUC based on needs. Hope this helps!