I put 30 blue hermits in my 40 gallon after I cycled, and they do pretty well, but the shell-addition thing can not be stressed enough.
Get LOTS of them. I added 10 larger shells to my system, and still had some of them die off. It is almost like they decide they need a new shell, and if they don't find one that second, they die. (this is an exageration, of course, but that is what it seems like)
Also, make sure you have algae for them to eat. I have a nano-reef in the basement (not doing well) and there is not enough light down there to keep the algae thriving, and most of my hermits have died off from starvation.