On the surface, the Forge is pretty straight forward. Create fusion coil, hurl at friend, repeat ad infinitum. However, there are some other things that you may choose to make yourself aware of if you wish to master your tools.
Inventory and Budget
The number of objects you can place in the Forge is ultimately limited by two things: Inventory, which is the number of a specific object that you can place, and Budget, which is an overall total that keeps things in check.
There's actually a third limit that kicks in when you've got a whole heck of a lot of objects on the map, but really, you need to be looking for trouble to bump into that one. The Forge will just squawk at you and give you a polite error message if you try to go above it. Don't bother, it's not that spectacular.