I have a couple Zebra or Left Handed Hermits in my 15Gal tank. I think they are pretty neat little critters. I also have four or so Trochus snails. The hermits haven't even bothered the snails at all. I do think it helps to get smaller hermits and snails that are a little larger. If you provide the hermits with plenty of new homes(extra shells) to choose from. You may not have any problems. Granted there are always some problem cases, but you do have the sump so you are all set...
just my 2 bits...