You can max out the allignment of your crystal at low levels, but it also levels in power with you, meaning that the stronger you get, the stronger your crystal gets, so the answer there is more or less a no.
As for the second, I think that the question sorta answers itself. If putting a darkside crystal in your saber makes it restricted to darksiders, and then putting in a lightside crystal, which makes it restricted to lightsiders, wouldn't that mean that nobody could use it? I've never tried it personally, but I would assume that it either never lets you wield it, or it'll only let you wield it if you're neutral. That's the real problem with this series, is that it always makes you choose one side or the other.

__________________
Get up, stand up. Come on, throw your hands up.
If you've got the feeling, Jump across the ceiling.