What is your oppinion on peppermint shrimp
Great animals. Sometimes they are a bit of a PITA in smaller tanks (like 5gal and under) if in large quantities. Short of that unusual situation though, they are usually nocturnal and shy. Taking out Aips requires quite a lot of them sometimes; they are unlikely to exhibit the behavior with <4-5 in most tanks, sometimes requiring more in bigger tanks.
Species ID first! Never buy a crab without an ID to at least the genus level. I have no idea what a "red crab" is...there probably a billion crabs that are red lol. Maybe one or two are good reef inhabitants and the rest are no-nos. For example, things I can think of that contain red species (or species that have a lot of red on them):
Mithraculus, Chlorodiella, or closely related -> great stuff*.
Uca & other fiddlers -> not fully aquatic, so bad for a reef tank, but sadly show up in the trade anyway. Often it's females, so the giveaway big claw isn't there.
Graspoids -> nasty fast predators, great fun in a species tank but a devil in most other tanks. Successful cases in community tanks usually involve pretty big tanks.
Anything else -> leave it at the shop unless you have a species tank for it.
*Mithracids and such are great stuff
when properly fed. No crabs are herbivores and the vast majority of bad Mithracid crab behavior results from people trying to keep them as grazers rather than as the omnivores they really are. A crab that wants meat and can't find it by any other means than predation will eventually explore that option.
Also, what is tyour take on Cerith snails
Good for the substrate. They go where the tastiest munchies are though, so some people get them for the substrate and then grumble about them spending all their time on the rocks/glass instead.
Gotta be careful combining them with some hermits (namely Calcinus species) but otherwise good animals, although you also have to be careful with the species of Nerite as well. A lot of them are tidal and like to leave the water, so in uncovered tanks with high waterlines they can end up escaping. I keep my waterlines very low in my Nerite-inhabited tanks.
Nerites are also not very effective CUC animals compared to larger Turbinids like Turbo, Tectus, etc. species. Itty bitty mouths are just not as good at cleaning large areas, so it takes a lot of Nerites to do the job of a single Turbo. And finally, they also lay little white calcareous eggs all over everything. Flat surfaces are a favorite place to put them. So, if you want totally clean glass...lol.
I'm rather hard pressed to find info if they will eat my coralline algae or not
They will not. No grazing snails are really equipped with the tools needed to make a dent in thick, calcareous algaes. Sometimes they'll be able to peck at little patches on the glass, but that's about it. Sea urchins are typically employed as coralline eaters because they can literally eat rocks by grinding away with their teeth. Snails mainly graze by "licking" surfaces, which is pretty ineffective at dealing with hard stuff. It takes an awful lot of licks in the same spot to have much effect on anything calcareous (and so my glass gets covered in annoying speckles of the stuff even though my tanks are full of snails lol). Sometimes big grazers can do damage to calcareous macros like Halimeda, but that usually happens when they're big enough to get their mouths around portions and start to "chew" more than "lick."