I beat up some cotoneasters this season to learn how they work. They can take some abuse, but are definitely a "one insult per season" plant. They will punish you with die back if you go too far.
You'll want a bigger pot to grow this out in, and I would recommend that you slip pot up to a large nursery pot in the spring to encourage growth (more of a light slap than an insult this way).
Trim a little bit off the bottom of the root ball and lightly comb out the outer perimeter of the root ball, but don't screw with the roots in any major way.
If you're going to hard prune roots it's best not to prune much foliage that year.
If you're going to prune back hard, do it early in the season and then just let it grow out. They don't respond as well to mid-season pruning. It's likely it will take 2-3 seasons to recover to a stage where you'll want to do any further styling.
After the initial styling, I'd just trim back the most vigorous branches each spring just enough to slow them down, but let it fill in until it starts to look like a bush again. After the 2nd major styling a few years later, it will probably start to really look like something.
They do grow relatively quick, but the growth habit is long and thin, so they need to put on some foliage and grow fairly uninhibited for a while to develop trunk/branches.