I have a gen 2 g21 that I recently added a rail to the bottom of with a surefire adapter (clamps to trigger guard). It works well and holds snugly to the weapon, but now of course the weapon won't fit an any but generic holsters.

While I expected this, I was debating cuttin off the tabs on the rail adapter that the screws mount thru and just epoxying the rail to the bottom of the frame. Question is, that would cover the serial # plate and would that be illegal, or not??

thanks in advance

Not!

You can't alter or cover the serial numbers in any way.  

I would agree that you can't cover or alter the serial number on the frame because in the US, that is the part where the serial number is supposed to be.  In other countries the serial numbers are officially kept on the slide and on others, it is required to be on the slide, barrel, and frame.  That is why Glock puts their S/N's on all three.

Aside from covering the serial number, I think that by gluing the adapter on the there, you are altering the flexibility of the frame and this could lead to cracks or breaking of the frame.  Glocks are designed to flex a certain amount and stiffening them like that could cause a problem.  Just gluing a piece to it like that is way different than redesigning the gun with a rail molded into it.