All the better. That's two less things from the core rules the DM has to worry about denying players.
I probably shouldn't mention, then, that shifters would probably be out since lycanthropes don't exist in Dragonlance. Of course, one can change that fluff.
If I recall correctly, knights could also be promoted into the next higer order (Crown to Sword, Sword to Rose). Would that be accounted for by multiclassing?
Right, you can stay in your order or progress to a higher order. So you can be a Crown Knight, or move up to Sword. From there, you can remain a Sword, or move up to Rose.
You can account for that through multiclassing or hybrid classes, or you can just stick with your base class and roleplay it.
For example, let's say you wanted to play a Rose Knight who exemplified the best of the Knighthood. You start out playing a warlord. You know that before he becomes a Rose Knight, he must pass through the orders of the Crown and Sword. As a warlord, he already has everything he needs to represent being a Crown Knight. As he progresses into Sword, he can take a paladin multiclass feat or perhaps skill training in religion. Then he can advance to Rose.
I think some of it, too, revolves around roleplaying. Perhaps you are playing a paladin who wants to become a Sword Knight. He starts out in the Order of the Crown, and his superiors quickly realize he has a higher calling. One quest later, and he's ripe for being a Sword Knight. While we can use rules items to represent the character here, we don't need to.
Hope that helps.