Starting torque will depend on a number of factors about the barrel and muzzle device. What are the details? I can give a recommendation for a starting point but, it would be just that, a starting point. I did misunderstand, timing w/shims to align the hand guard to the barrel nut shouldn't cause an issue.
As far as barrel extension to upper fit, that depends on just how loose it is. If there is a lot of slop to take up, you can use some shimming material. I try to avoid uppers that fit this loose if at all possible. It's somewhat of a hassle to deal w/the shimming material if you ask me. Over the years we've also built the extension up and then turned it on a lathe until it fit snugly then bedded with some Loctite 620. Truth be told, as long as the upper isn't really loose, bedding w/Loctite 620 has yielded results that are just as good. The trick is to avoid a really loose upper if you can (this may be easier said than done, especially in the current market).
