As everyone else has said, wear what you want, play how you like :-)
I've found some aspects of military knowledge are useful, others less so, you will get shot by 12 year olds though... Happens to everyone :-)
Wouldn't be massively put off by MILSIM either, even the more 'hardcore' games are really just man crèches with toy guns and slightly more organised teams/objectives. You certainly won't be pulling stag for 6 hours in the pissing rain after 2 hours sleep under a poorly erected basha, not unless you really wanted to anyway!