I'm in the same boat right now. I have a pair of Klugei and a pair of German Blue Rams in my 55 Gallon. So far I haven't run into any major aggression issues, however I haven't had them together that long. I read a lot about this and found most people advised against it due to the aggression during spawning. However, that being said, in my case the Klugei will spawn in a cave and the GBRs will over a flat surface (i.e. rock), so at least they shouldn't have to fight over WHERE to spawn. I'm assuming there could be problems though if they choose the same area of the tank. Unfortunately I don't have much to offer other than that, but if you do go with both pairs just keep in the back of your mind if the aggression issues get out of hand, you may have to re-home a pair. Hope this helps!