I asked the same question the other day and it turned out to be a big no no, the crabs would pick off the fish while they were resting on the bottom, shame though, would be a realy interesting addition to the malawi tank
are both rift lake crabs BUT... in the hobby they have been notorious for escaping tanks so lids MUST be free of holes. not to mention when crabs molt and they must molt to grow the surrounding fish enjoy a buffet of crab feast! they're extremely vulnerable to attack then.
it's advised if you want a crab in your tank....do just that...a crab only tank! lol much less headache that way. you can enclose the tank and theres no risk of molting death
I've seen a mbuna on molting crab buffet and it aint pretty folks!