Any opinions of these snails? I have a 90 gallon tank, and got hold of 100
snails. I want to build a cleaning crew of turbo snails, emerald crabs, etc.

I think they are excellent snails, keep the sand stirred and quickly devour
detritus

Are nassarius snails good critters to put into this mix?

Definitely

If so, what would
be a good ratio?

I'd put in at least 10--20 in depending on how much sand & bioload you have.
Keep in mind these are not snails you'll see on the aquarium glass - they are
strictly sand dwellers IME.

HTH,
James