Ive just purchased a rooted 4ft Fraser Fir tree ... I intend to keep it potted...
This is a Fraser Fir, growing in a pot...
I actually bought that, growing in a small pot, from Homebase, several years ago. For the first few months I just kept it in the pot it came in. I have potted it on, into slightly larger pots, a couple of times. I can't recall exactly how long I kept it in its original pot.
I prefer to use John Innes No 3 compost. John Innes is soil based, so that gives the pot greater stablitly, and makes it less likely to blow over. No 3 is the compost to use for trees and shrubs. When potting into a larger pot you don't actually need that much compost.
I tend to neglect it. I give it some tomato feed occasionally. I know it's happy because last year it had a few pine cones on it.