I am new to HOTT, so i want to be sure, i understand how mages work.
My reptiliad mage element will be fine in a group with his Gilla worm hordes, because he also uses 2 PIPs?
Yes the Group moves for 2 PIPs.
If he charges into an overlap, he can still first put a spell for 2 PIPs on the enemy and then be used as overlap?
Yes you can cast if not in contact (and corner contact does not count as contact). You can cast into a Close Combat.
The other player is acting and puts a spell on my mage. I win. The other Mage is bespelled (without me using any PIPs)?
Yes a defending Mage can inflict negative results on an attacking Mage.
As a variant we allow Mages to put as many PIPs as they wish into a Spell with your CF for that spell equal to 2XPIPs BUT if you roll less than the number of PIPs you put into the spell you become "depleted" and two depletion turns you into an Old Hag or Frog.