Interesting questions, I'll do my best to answer them:
#1: I didn't think that there was a problem balancing the PC's, although one of the players suddenly upped and decided that he did. You will notice that M'ir the monk/savant is no longer present... his player felt that "his character wasn't pulling his weight in the group". Now, he had the best AC of anyone, and was one of the few characters to not even "nearly die". His level of monk was a good synergy with his Wis, contributing nicely to his AC, but it slowed down his rate of power use. He was comparing what he would be at his next level (mnk1/seer6) with what the Sor7 would be doing and was disappointed that he couldn't throw lightning bolts like the sorcerer. He had a character with great versatility but very little direct power in one direction. The other "psionic" character, K'tan is fine at present.
#2: I've not had any trouble balancing "psionic" NPC's or monsters. As you'll see in my next answer, I treat it as just another kind of magic. Actually, the answer isn't strictly true... I find it as difficult balancing encounters as it always is
#3: I do have some psionic house rules (over and above the standard "same as magic" option).
a) No psionic combat at all. I've completely ditched everything about direct psionic combat. This is because it doesn't fit with my vision of using the psi rules as just another kind of magic, and because I think it is more of a hindrance than a help to psionic characters! Mind Blast is the only thing rescued, and it has become a 5th level Telepath power.
b) Psions get a bonus metapsionic feat at 1st level and every 5 levels.
c) The
Concussion series of powers is a ranged touch attack with no saving throw, and can affect objects. This has caused plenty of fun as slightly missed concussions blew chunks out of walls, knocked down doors and such. It also means it can be used against undead (which technically, the standard book version cannot!?!)
c) I have my own "power scaling" variant. There are two elements to it basically. Firstly, you can put less pp into any damage-causing power which you can manifest. Thus a 6th psion could manifest "burning ray" with 2pp for 2d6 damage, a 10th shaper could use from 1pp up to 9pp when manifesting "burning shroud"
The second part of the variant is that higher level powers use bigger damage dice. I'll admit that this isn't playtested thoroughly for the very highest levels, but I'm running with it for now.
The basic principle is that higher level powers have an increased damage die - they make more efficient use of the pp expenditure. At any given level an effect with a fixed radius uses a lower die size, and one with a variable area like a cone uses a die two sizes lower. The basic rate is summarised in this table. Sever the Tie is an exception because of its very specific target "audience"
<table border="1" cellspacing="0" cellpadding="6"><tr align="center"><td><b>Spell level</b></td><td><b>base die size</b></td><td><b>fixed radius </b></td><td><b>variable cone</b></td></tr><tr align="center"><td>1-2</td><td>1d6</td><td>1d4</td><td>-</td></tr><tr align="center"><td>3-4</td><td>1d8</td><td>1d6</td><td>1d4</td></tr><tr align="center"><td>5-6</td><td>1d10</td><td>1d8</td><td>1d6</td></tr><tr align="center"><td>7-8</td><td>2d6</td><td>1d10</td><td>1d8</td></tr></table>
edit: since HTML is turned off!
Spell level | single target | fixed radius | variable cone
1-2 | 1d6 | 1d4 | -
3-4 | 1d8 | 1d6 | 1d4
5-6 | 1d10 | 1d8 | 1d6
7-9 | 2d6 | 1d10 | 1d8
The rationale for doing this is described in some detail on my web site.
I've not decided precisely what to do with the 9th level power "detonation" yet - but it will be a while before I have to worry about that.
So that, basically, is the house rules which I use for "magery" - my version of psionics.
Cheers!