Well, the shrimp the OP posted about are more aggressive. My fire shrimp stay out eachothers way in my 100 gallon but occasionally they will go at it. A coral banded can be very aggressive. You have never heard of the rule I posted because I made it up. Its just from my experiences watching interactions with shrimp and how much space they usually "claim" for themselves. If they pair up, great. If not, they do fight, especially if there is food in the water. There are no hard and fast rules, just generalizations. The more "different" the species in both size and body type, the more likely to get along, unless you have a particularly aggressive individual. Same goes for fish. Anyway, I digress.....