Definitely add a clean up crew before fish. A lot of people recommend making sure your ammonia and nitrite are at zero for a few days or a week, then add the clean up crew (snails/hermits), then wait another week or two (while testing) before even adding fish. Some people say hermits will attack and eat snails, but it's up to you if you want to take that risk. If your hermits kill $50 worth of snails, I'm sure you wouldn't be happy. I have a few dwarf blue hermits, and they seem OK, except for one that's now in solitary confinement for killing a snail. I think he may be a different kind, as he's larger than my other hermits.
A mix of snails should be good. I started off with a few, and add a few every now and then.
I'd say put any fish into the quarantine tank before you put them in your main tank. It's like playing Russian roulette, otherwise.