The problem is that SH isn't ready to bed the player until Act 3 because of her Shar stuff. Karlach having to wait on SH before she can be touched would be brutal tbh.
Karlach and Lae'zel on the other hand would absolutely be compatible. It's not like Lae'zel doesn't compliment Karlach every chance she gets lol And they wouldn't have to wait on anyone.