Mage + ToFS
 

I had a lot of fun in ToFS on both my enchanter and my necro. I didn't go there though until both were in their 50's just to mess around.

Can a mage level there? If so, at what level could I start on first floor?

I'm guessing at 34/35 you could manage most of the pulls. Second floor will be a problem since they can charm your pet up there. Had them charm my Emissary of Thule pet on my necro multiple times.

Is it possible to invis-poof a pet that has been charmed? Or just dispel its charm?

Nope. Once it's charmed it's no longer yours. As a necro, I'd just charm the mob that charmed my pet and it would die. If you kill the mob...it's weird. Usually they start attacking themselves until they die. I dont know if an enchanter could recharm them, but as a necro, I couldn't.

That may be something to bug report. I recall when I was on the 2nd floor with my necro the mobs there had zero trouble charming my 49 pet. In fact there were a few times I went up and would find one of the NPCs with an earth or water pet from a PC mage that they must have charmed it from. There was a 49 earth pet there on day.

Anyone have any insight into whether this is classic or not?

TOFS NPCs didn't start charming until some NPC spellcasting updates made during Luclin. The evidence and bug report is here.