responses to multiple questions:
i have 2 fire shrimp, 2 cleaner shrimp, and 3 peppermint shrimp. one fire shrimp is very agressive towards all the others. the peppermint shrimp are usually the most shy, the fire and cleaners are usually walking around all over my rocks.
as for the high fin goby... mine is a total wuss. it hides all the time, basically only coming out to eat. however, at certain times he will come out and survey the area around his burrow... usually only traveling about 2-3 inches from his home. no, i do not have a pistol shrimp in there with him yet.
for shrimp gobies, i have heard good things about the randalli's goby ( http://www.liveaquaria.com/product/p...cfm?pCatId=197
)... but my opinion would be to get one of the sandsifting gobys, as they tend to be out in the open more... well, thats based on my observations at the store.
on a related note, if you are concerned about the stability of your rock work, you can build a pvc
structure to suppor your rocks, and bury it in the sand. the pvc
will hold up your rocks and still permit your livestock to burrow in the sand below the rock without jeopardizing the strucuture. i used 3 inch pvc
couplings, and stood them up on end. they are just deep enough to be covered by my sand bed.