It all depends on the crabs you wish to purchase, in most cases the crabs just threaten the fish if they get too close, they raise themselves up and wave there claws, I've not lost a fish so far due to crab attacks.
All crabs are master escape artists, the slightest gap in a tank lid and they are gone,
they will climb up anything,
my reds will sit quietly eating a piece of cucumber, then climb up to bask on top of my filter tubes. I have some plants that cover part of the water surface, they quite happily sit cleaning up any food that is stuck in the leaves.
You can buy underwater crab dry shelters, in other wards it is a place the crab can sit in air whilst underwater, but these are a pain to clean and upkeep.
The Corries should keep out of the crabs way, but the loaches just might be winkled out of the sand/stones or substructure ie they can't escape as easily as normal tank swimmers, I can't put any snails in with my reds they are experts at eating the meat from the shells.
It is completely up to you if you wish to introduce crabs into your tank, A crab will grab a fish if the opportunity arises, but they mostly pick on the sick or dying.