Another thing I will add is that I've always looked at 50xp to be 1 level in comparison to D&D (3.5). "Leveling" every other or third session. Now I don't know alot about d6 or d20 star wars, but you said level 18? This would put you, in my opinion, in the 900xp range. Which is what I'd expect PCs to be after playing for years. My table played frequently over the last year and they are just now approaching 300-350 XP.
You mustn't be giving very many XPs then. A basic human "knight level" character is near 300. (260 to be precise). 110 in starting XP plus 150 for "Knight Level". With a moderate by the book rate of 20 XP per session,it should only take a 100-110 XP starting character about 12 sessions to reach 300. If you're playing once a week, that means within three months of play, a starting character should be at 300 XP. He should hit 1040 XP in one year (52 weeks). Even if you use a slower advancement rate of 10 XP per session, that still means 300 XP total within six months of weekly play, and 1200 within two years.
I don't think the amount of XP is a problem, you and your GM can find a compromise. If it's a GMPC doesn't really matter what his XP level is.
I want to play Korath as a regular PC, rather than as a GM character, Particularly until he's tied up the "loose ends" with his nemesis. Besides, I'm not experienced enough with this system to even attempt to GM it. I don't have a GM yet either. Ideally, I would like to get him down to the 2000 XP cost, lower if it doesn't require gutting his D6 and D20 abilities and Force powers.