Put them (and anything else wet that isn't a fish) in a fishless tank for 60-76 days and keep them fed - that allows time for any fish diseases to die off, and gives time for you to observe and deal with other issues you can observe (like bubble algae or hydroids on snail shells, parasites in a shrimp, etc.).
^This is exactly what I do. I’m keeping three 10 gallon QT/Observation tanks for all new livestock - a medicated fish QT, a coral observation tank, and a third observation tank for motile inverts like snails, shrimp and crabs. I’m running this tank with just a heater and an AquaClear HOB filter, no ATO currently, no light, except ambient daylight and a little spill over from the coral observation tank that’s next to it (I keep my fish QT greater than 10 feet away from all other tanks to minimize any pathogens or parasites that can be passed by airborne water droplets (<Yes, this is a thing!)). My coral observation tank is bare bottom with a frag rack, but for the other inverts I gave the them an inch or so of dry sand and a few pieces of dry rock for real estate. I feed them almost as if I’m feeding fish, I give them frozen as well as flaked and pellets, and I encourage a little algae to grow in this tank, and they seem to do fine.
There’s some debate over how long you should quarantine motile inverts until considering them safe to add to your display tank. Some say that crabs and shrimp only need to be held until they molt, others say to hold everything for 30 days, many say 60 days, and 76 days is also commonly discussed. I go with the 76 day time frame, I just feel it’s that much safer, and I don’t mind being a little extra patient. I’m trying to hold new coral frags for the full 76 days as well before adding them to my DT.
And just to mention it, for the fish I’m using the R2R approved QT protocol found here:
https://www.reef2reef.com/threads/current-quarantine-protocol.825055/
The QT protocol found at HumbleFish and Reef is also very popular, but I chose the above method because the authors are right here on R2R and available if you have questions (HumbleFish also has a help forum, but I figured I’m already a member here).
I hope that helps, good luck!