There is your first problem - the level disparity. If you team up with your LVL 33 friend, you either won't get much XP (if you do missions for the LVL 11 characters), or you and the Knight will stand around and try not to die (if you do missions leveled for the LVL 33). I would recommend you tell the LVL 33 to roll an alt and catch up with you on Coruscant.
Right, yeah I get that about the level disparity. I was more thinking in terms of myself and the Knight; the 33 was just going to run around with us till we caught up.
Class quests go on to end game, yes.
However, all planets have planet quests that can be run by anyone. Talk to anyone with a yellow triangle over their head and they will give you their quest. You can tell the difference between a class quest and a planet quest by looking in your quest log. (They will be listed as "Class" or under the name of a planet.)
What about getting to the quests? I had a series of quests on Nar Shaddaa that I was able to share with the Knight, but the taxi routes were not open. He had to run around doing random other quests till they opened. Is there another work around that we haven't seen?
You can't abandon them. You can level to 50 without doing them, if you PvP, run FP, or do space missions. However, you won't get your companions, meaning your ability to do crafting will be reduced. You'll have to go to planets anyway to get crafting materials, though (unless you buy from the GTN).
That's kind of what I figured. Oh well... maybe BW will make grouping more flexible in a future patch (HINT!).