IME, I had issues over my first tank's 6 mo, keeping inverts alive. I have no clue why. Nowadays that doesn't happen and I think it may be due to a more stable water chemistry (older and larger tank system). I also learned the hard way that I keep my water too warm for Margarita snails.
Also, crabs are *******s. Ceriths and Astreas proliferate in my crab-less tank. I depend on crabs in my tank with crabs who end up killing half of the added snails over time. The rest are faster, wiser and generally larger like turbos.
How old is your tank? I say add your CUC, let it play out however it does. Learn what species thrive and work hard in your tank then play to that.