It depends on your unit. Some RSMs and CO/Ocs will let you get
away with it if you have a valid reason, some won't. Also, depends
on what the ICT is lah.
There are cases where rock singers have been asked to just tie
their hair up or make sure it looks tidy.
but then if you gonna be running up and down hills, digging
trenches, or doing fire and movement.. Are you really sure you want
to keep your hair long??
Erhhh... the "high salary = less call-ups" doesn't really apply
anymore. Trust me.
As for disruption, yes you can. Supposedly they can call you up for
service till you at 45 (for men) and 55 for (officers).
But if you don't make the required number of icts, they will just
call you back more often as your time comes. You then get posted to
different units, and often times, you get stuck in some lousy unit
doing "sai kang". Trust me bro, it ain't worth it if you can help
it.