Th Th ... some other points ...
In 1e and 2e there was such a thing as terminal classes.
You could take Elf to level 8 maximum, in class Y.
You could take Halfling to level 4 in Class Z, but uncapped if a Thief.
WHY ?
No that's the wrong question. It's even wrong to ask the question.
Gygax said so, and so it was ... and it was "good".
3.0e and 3.5e continue this History, in the thinnest of fashions.
Having a "favored" class is this leftover.
4e goes ahead and ditches the idea altogether (but it has it's own quirks).
So anyways ...
if you accept the claims made in the previous posts,
about the power level of Core Wiz or Cleric or Druid 20,
then it doesn't really matter what kind of multiclass fun / nonsense happens.
If the DM
insists on keeping it,
well you can either build around it,
which requires having a good idea how it works ...
[tangent] the math gets complicated in a hurry, and the rules have never been fully figured out [/tangent]
or
ignore it completely and make a good build of less multi-wacky stuff.
Pixie = carnivore ...
Pixie has a rather good racial hd. You could always give it that.