blue legged hermits, 6 x red legged, 3 x rock
You should probably try to get species IDs on these. The first will almost certainly be Clibanarius tricolor, the second may be Paguristes cadenati (a potential risk to the blue legs) or Clibanarius erythropus (less of a risk), and I don't know what you mean by "3 x rock." Also, what size tank? Mixing hermit crab species is not a great idea unless the tank is huge and the hermits very sparsely stocked.
I have 1 piece of live rock which is covered in green algae but they don't seem interested in that piece all others have been cleaned by the crabs but not this one piece. Is there something wrong with that piece that I should be worried about or look out for ?
It may be a type of algae the hermits simply aren't interested in. Most hermits prefer meaty foods over algae, which may be part of why they snacked on your snails if alage was the main available food for them.
How can I tempt them to come to the front of the rock instead of hiding ?
A piece of popcorn shrimp or krill tied firmly to a weight (so the first one to arrive can't run off with the whole thing) and put near the front of the tank usually works. Sometimes doing this with the lights out may have better results if the hermits are trying to hide from something during the day.
I am looking to add other crabs 1 or 2 of the following
Teddybear crab
shameface crab
sallyfoot crab
In general (with the possible exception of the Mythrax genus, although I'm not too sure of the hermit safety of those either), adding true crabs to a tank with hermits is a bad idea unless the crab is certain to stay very small in relation to the maximum size of the hermit crabs. Even fairly small crabs can attack hermit crabs when they molt. Similar damage can be inflicted the other way around, but molting crabs are usually not easy for hermits to catch unless the crab has become compromised in some way.
With Clibanarius tricolor (bluelegs), the shameface crab will be a risk. If the crab gets hungry and the hermits aren't wearing oversized shells, all it would have to do is pick up the hermit, stick a claw in the shell, and start pulling legs off. The teddy bear crab may also be a risk, but it would likely depend on species. I've seen a number of hairy species go by that common name, some of them being larger and more aggressive than others. I am not sure of sally lightfoots. From photos I've seen of their maximum sizes, I wouldn't add them with any small hermits for similar reasons.
are they ok with shrimps - peppermint (x2), cleaner (x2) & blood (x 1)
The hermits should be ok. Not sure as to the true crabs.
