Hermit crabs are scavengers. As long as there is detritus, uneaten foods and various types of algae about, they will find a meal. There is no need to worry about them going hungry unless the population is larger than the food supply. Stick within that, build your crew slowly and you should not have a problem.
For a 100 gal
tank, you could quite easily have a mixture of both the blue and red. Start out with about 10-15 of each so as not to overdo it. Personally, I like zebra hermits as I have found them to be the least aggressive and do not generally prey on snails as long as "housing" is otherwise provided.
Don't forget the snails....