No shortage of oppinions here, but I thought I'd add mine too :lol:
I love nassarius (not sure I spelled that right) They're carnivors, so IMO/E they do a good job taking care of leftover bits of food. My feelings on hermits are mixed, I have about a dozen in my 30g , I think they tend to keep my *pods to a minimum. I have no hermits in my 50g that is more recently set up, and lots more *pods (now, this could be just from it being a newer system, time will tell). I have substantially more Nassarius in the 50g because of the lack of hermits.
One thing to watch is that you don't get more hermits than your system/feeding practices can support, because there is the risk that they will kill your snails for food/housing (which sometimes they'll do for no reason).
If you do get more red-legged hermits, make VERY sure they are Scarlet, there is another red that my LFS called "mexican red-legged" that is a real trouble-maker and not reef safe (I got one somehow and had to trap it after it split open a featherduster and ate it!) Scarlets have bright red legs and this mex. was more of a maroon.
If you're looking for a "safe mix" err on the side of more snails. IMO there's not a big problem with too few hermits, but you can certainly have too many. And the opposite is basically true for smaller snails (within reason). Overdoing crabs will cause them to eat things they shouldn't, whereas overdoing snails will possibly starve one or two. (Which of course should be avoided, but at least you don't have to trap-out a sneeky troublemaker, like I did w/the Mex. hermit, that was quite an ordeal and I lost a $20 featherduster in the process.) I think this is the biggest contributing factor to why I have a "hermit-free" 50g.