It will probably fill in all the empty space but I don't think it will get to the point of overcrowding or choking out the geraniums. They should grow together nicely. You can also train the ends to spill over the edges of the pot. If for some reason you want to keep them separated, you can trim the ends but this will promote "branching" of the plant.

It will. Try to train the main stems away from the geraniums.

Trim whatever crowds, when it crouds.