I always get 5-6 mags per pistol. Just old habit I guess. I don't like having only 2 mags. Some day, it may be hard/expensive/impossible to get the mags, so whenever I purchase an auto pistol, new or used, I always order plenty of mags for it.
And yes, the full-size P99 mags work great in the compact, but like someone else said, the size and feel is much different.
-Jeff