I don't think you'll have any problems with them getting along. They'll likely avoid one-another altogether and find their own little hiding places in the tank to stick to as a home base.
I would be more worried about supporting those crabs in a 6 gallon. I have struggled with both of those species in the past (1 of each in a 9 gallon), and they require quite a bit of attention and spot-feeding. Your system likely won't be able to support them without that. There just isn't enough particle matter or detritus floating around in a 6-gallon for that, especially if you have a cleaner tank.
I'm not saying it's not possible, I'm just saying it will require extra effort. They are both really cool little species and fun to have, so best of luck.