The bonuses gained from Stone Haven Outfitter's abilities are in addition to any bonuses given by the Equipment.

Equipped creatures will get just +1/+1 from Stone Haven Outfitter, no matter how many Equipment are attached to them. Similarly, the last ability will trigger just once per equipped creature.

In this context, "equipped creatures you control" and "equipped creature you control" refer to any creatures you control with Equipment attached to them. In the highly unusual case that Stone Haven Outfitter becomes an Equipment, the meaning of its abilities doesn't change. The bonuses continue to refer to any equipped creature you control. They aren't bonuses applied solely to the creature Stone Haven Outfitter is attached to.