Can we add ember add-ons to a discourse plugin? (like ember power select)

So you’re literally looking for trouble. I’d recommend not considering any addon until you find that the existing resources don’t meet your needs.

But you don’t know that it’s going to be compatible with how discourse solves that problem today or in the future. If you want to develop for Discourse then looking at examples of how things work in discourse is going to be the way to go.

Don’t be the guy looking for his keys under the lamp rather than under the car where he dropped them because you can see better under the lamp.