YnK: I'm assuming you're playing vanilla PST, 'cause IIRC Qwinn's fixpack makes TNO get the Fighter's 1d10 HP on level up regardless of his class, and you no longer keep your Fighter THAC0 as a Mage so your proficiencies are relatively worthless there.

If you ask me, staying in the Fighter class isn't worth it at all, even in the vanilla game. All you achieve in the end is losing extra Mage levels, meaning you hit Mage specializations far later than you should. And with TNO's regeneration and auto-revive, HP doesn't matter much: as a Mage, you'll be using ranged spells most of the time (since spamming Magic Missile can kill pretty much everything), plus you get shield spells and AC bracelets to protect yourself.

Since you want to get double-specialized before a certain point of the game (won't spoil that), and you'll run out of quests way before that, prepare for extensive level-grinding on respawning enemies (there's a place that'll let you do this pretty fast, though).

kmonster: Fighters get a new proficiency point every 3 levels, thieves every 4 and mages every 6.
You can only get an extra proficiency point when you reach a level not reached in anothre class before, so if you're already mage 9 it's pointless adding a fighter level for it.

If you aren't already level 9 as mage, you can get the proficiency point if you reach fighter level 9 first, but 125,000 XP is a high price to pay for only one proficiency point and one thac0, it's a third of a main (mage) level up which offers an extra attribute raise, extra spells and casting level. But I guess for your playing style it's the right thing to do since you can simply grind away the disadvantages.

You can only use the fighter weapons when you're a fighter, XP gained while using them will only add to your fighter XP.

YnK: Regarding HP: normally, Fighters get the maximum of 10, Mages 4, and Thieves 6 (and it's supposed to be a random number from 1 to the maximum, unless you have the MaxHP tweak installed).

In the vanilla game, TNO receives HP according to his current class (if his level in that class is higher than in other classes). In the patched game, class doesn't matter, levelling up the highest class always gives him up to 10 HP, since that's what the manual says should happen; presumably, it was meant to be like that specifically to prevent metagaming (that is, gaining Fighter levels for the sake of HP even if you plan your main class to be Mage). Levels past 20 only give 3 HP.

While Fighters receive THAC0 upgrade every level, Mages only receive THAC0 upgrade every 3 levels. In the patched game, no matter how many levels you gain as a Fighter, switching to Mage adjusts your THAC0 according to your Mage level. At level 20, that's some 13 points of difference. So if you want efficient melee combat, you'll have to switch to Fighter class, even though you can still use daggers as a Mage.

Gambits: Would you happen to know why I'm only getting 3 hp from lvl up as a mage? I have 18 con so I thought I should be getting 1hp+4 from con bonus.