I can't say for sure about the tac light on the USP compact, as I haven't had the chance to try it on one yet. I have shot a compact USP quite a few times, and thought it was an excellent pistol. I am extremely pleased with the full sized 45 USP, as far as reliability and accuracy.
The compact USP is a fairly big pistol, so I don't think it would be a great choice for a concealed pistol, although the accuracy, reliability, ease of use, and knock-down power are all excellent.
The fullsize USP H&K magazines are 10rd capacity, usually sell for about $35 new, and there are some H&K 12rd magazines available, but they are expensive ($85-95 each). Just tried a Pro-mag brand magazine, and fired 200rds without any problems. They sell for about $15.
Hope this helped. Andy