I don't think this one works anyways. The main reason for this is the Wild Shape doesn't actually change your type. Now, presumably, being a 'true beholder' actually requires you to be an aberration? This is part of the reason why Planar Shepherd efreeti can cast Wish upon themselves, incidentally, they're still nongenies when Wild Shaped... It's just in this situation it comes back to bite us in the ass.
Mind you, if someone came up with a way around that problem, you would still not really be able to combine 10 levels of Beholder Mage and 10 levels of Planar Shepherd (Or rather, 9 levels of each since that's when they get their capstone abilities). A somewhat sneaky way around this problem, assuming you again qualified as a 'true beholder' would be to also take the Divine Minion template. This gives you the Wild Shape ability of an 11th level Druid. This qualifies you for Beholder Mage after level 1. The build would go something like this:
Druid 1/Beholder Mage 9/Planar Shepherd 9
In this case, you'd be using your Beholder Mage levels for all your spellcasting, with your Planar Shepherd levels just there to give you some Wild Shape goodies. Incidentally, does anyone know if Divine Minion stacks with a Druid's Wild Shape ability? Does anyone have an authority for it stacking? If it does, I can't see how the template isn't mandatory for all Planar Shepherds, given how much it would increase your Wild Shaping ability by.