When you go overlapping collisions it tends to cause Unreal to get confused with its built in calculations, Because of this you can butt the collisions up against each other just can’t over lap. As for a gap, think of the size of the players you are using and if they can fit through the gap then it is to big. When I make my collisions within 3ds max I tend to make them just a hair smaller than the object I am using them on. It creates clipping issues are times but sometimes you can’t get away from it.
I hope this helps
Carl