I would honestly suggest CUC before fish, but that's me. My logic being that your CUC come partially from your live rock and are an incredible indicator of when your parameters fluctuate. My rule of thumb is minimum 2-3 inches of live sand substrate and 1lb for every gallon in the live rock department. The leeching of your live rock will cause it to push out the detritus inside and start your clean up crew, but also cause an ammonia and nitrite spike, be prepared.... Reef tanks take a long time to start up, and the tried and hands down most reliable method of cycling a reef tank is using a dsb
(doesn't have to be live, though it does help) and aquacultured live rock and a table shrimp in a nylon sack and can take up to 9 months to actually start propagating the proper environment for a reef, along with being a completely money draining hobby, lol.