I'm going to be putting a teleprompter on my tripod. The head of my Manfrotto 503 has a load capacity of 17.6 pounds. I want to put about 21 pounds on it. I dont plan on moving it much. Everything will be a fixed straight shot. That said, am I asking for trouble if I do that?

Where you may run into problems is trying lock in position, off the center of gravity. In other words if you want to tilt and try to lock into position. With too much weight the friction tilt lock may not hold. I am guessing it may not be an issue at 21 pounds, unless there is a top heavy arrangement.

I have used the 503 with Manfrotto legs to mount my home built gib. With weights and camera, that gib comes in at around 30 pounds. I have tried to lock the tilt in position, and most of the time it works, but if it gets out of line with level, the head will tip down despite be locked solid.

a quick look at B&H found this. Ikan TP-SA Teleprompter Stand Adapter TP-SA B&H Photo Video
I didn't look much further but I'd bet there are others that would work as well. A decent light stand or C stand and you're all set.