I'm not sure about the Vella tubes but you want to get automatic extension tubes which have electronic contacts to relay exposure information to your camera. Much more convenient. I use the Kenko tubes.
As for the macro focusing rails, there are the cheap ones on the auction site for about $50 which actually work pretty good with decent accuracy and build quality. Then there are the really expensive products from Really Right Stuff, Novoflex, etc ($$$) which are absolutely beautiful and extremely precise to use but perhaps overkill for most situations. I've heard that the Velbon rail is good.
With regard to losing image quality on your lenses, the Canon 250D diopter is a close up filter that is an extremely sharp, corrected optic. You will not notice any real loss of sharpness and you can use it with far less hassle than bellows.
Of course there is also the fact that although a lot of lenses will perform decently when used with extension tubes or diopter, there really aren't many lenses that have the edge to edge sharpness the a macro lens is going to have. So if you want the best possible sharpness you can, you should probably make sure you have a macro lens. My Canon 100 macro is razor sharp. And on your 7D will effectively be a 160mm macro which will give you more working distance.
One last thing I would suggest is that if the macro subjects you are photographing are stationary, you might want to consider focus stacking. This will allow you to use optimal apertures instead of stopping down to f16/22 for depth of field. You can successfully use your macro rail or manually change lens focus to create the stacks.