I think they'd be more believable if they just couldn't join a faction permanently. Let them not be able to communicate with most races, make their quests very selective only allowing them to communicate with feral types. I mean, it could really work, but if they just slotted them into the same race space they use now it would be very silly. More and more monsters are moving into alliance capital cities, and the horde is becoming a collection of races with very very different sometimes competing needs. Murlocks just wouldn't be a good fit and would end up feeling faction awkward just like the pandaren, tauren and draenei.