I would go lighter on the crabs and heavier on the snails. 20 Hermits are IMO
too many for that system.
Look into some Nassarius, Cerith and Turbo Snails ( the bumble bee are pretty, but limited in their clean up ability). These others are all an asset to any tank.
Nudi's are not necessary or desirable for a new tank. Chances are they would starve to death. They have very specialized diets.
CB shrimp are cool to look at, but a pair of Skunk cleaners would be better if you plan of having fish.
Please read through some posts about clean up crews on this site, much great info to be found.
welcome to AA