PP5, in regards to your questions.
You can have a firefish and a watchman goby as well. The watchman gobies are agressive towards other things in the tank that have the same shape as them.. so i would think that getting two watchman would not be the greatest of ideas, but then again, your tank is pretty large, so who knows. Also, blennies do fine with gobies, so i would say the lawnmower blenny would be fine as well.
And for the sailfin tang, they have a minimum of 125G. So i don't think it would be wise to put it in a 75G, even though the 75G is pretty big. Tangs in general usually just establish a pecking order.. i've never heard of them killing other fish.. but then again, i don't own a tang, so no experience in that department.
Be aware that if you do get hermit crabs, it will be hard to keep snails, since the hermits will most likely kill the snails for their shells. People throw empty shells in the tank thinking this would stop the hermits from killing the snails for their shells.. but the fact is, that the hermits have to find the empty shells.. and they usually find the snails first.. lol. Also, over time the shells get pushed to the bottom of the sandbed. Assuming you have a sandbed, and sand sifting critters. Also you should do careful research on sea stars because some of them eat coral.
