Here I will answer all those three questions happliy for you:
yes should be no probably.... geophagus are peacuful cichlids
sand is optional. you don't need sand but sand helps the geos swift at the bottom better. Gravel works to.
Not much aggression at all... the only aggressive you will have is between the geos
I have never heard of them getting gravels stuck in their gills, figure they would be smarter than that. Guess me and some of the guys around here are just really lucky to keep geos on gravel without any incidents.