Diversity in snails is my advice. I do think that you could use 10+ snails I think you should mix them up.
I think you could use three or four of these and get two or three of each and be very happy!
Hermits can be benificial or a disaster. Use at your own risk! I only use Mexican Redlegs because I have found them to be the least destructive.
Same goes for Emerald crabs. I would have at least one in your system but not until later when your system is well established.
I personally don't like starfish unless it is for ornamental reasons.
I hope this helps.