I've actually had a campaign with similar circumstances -- most of the characters were new, 8th level characters, but some of the other characters were old characters converted over from 3E. So we had a group of 8th level characters travelling around with a 13th level cleric, among other things.
Two things to help balance it:
1) Scale the XP awards for the lower level characters. Mr. L7 will be L7 for like 8 sessions, but the L3 characters should be, say, L5 by then. He should gain like 1 level for every 3 that the other characters get, until they've caught up to him.
2) Don't give the L7 character equipment appropriate for his level. Either start him on the same footing as the L3 characters, or actually start him lower. The "characters being defined by their stuff" concept goes both ways -- a character that isn't outfitted appropriately for their level is going to be much, much weaker than one that is. In the campaign I mentioned at the top of my post, we also had a L18 character travelling with the L8 party -- but she had so little starting equipment that she was actually less effective than the L8 characters.
3) What class is the L7 guy supposed to be, anyways? If he's anything other than a druid, wizard, or sorcerer, he won't have the killing power to tackle the adventures solo. If he's a cleric then he'll basically be a safety net for the rest of the party, and if he's a fighter-type then he's really just a big tank.
edit: Jedi Knight? Hmm. I've never played Star Wars d20, but I think what I said still stands.