I would think that the commonly available speeds of 6000-7000 rpm would be just fine. I'm building one two.. so I'm hunting around as well. The ones from radioshack are generally pretty fast... but you don't want them too fast to control.
If you have glass mirrors, a simple disk of thick plastic can help ..... just do the hole in the center of the disk, with the desired inclination, glue the disk on the pin and the mirror on the disk.
If instead you use the acrylic mirrors, you can do the hole directly on the non-mirrored face of the acrylic (paying attention to not go out from the other surface and not deformate it), then glue it directly on the pin.
just remember, more the assembly is centered and without vibrations, and more stable are the figures
The shaft on the pager motors are 1mm, you dont have to have round mirrors and the advantage of using square mirrors is that its alot easier finding the centre of a square than it is a circle!
Dont get too hung up on centre position as you have to have a little 'wobble' to create patterns.
To create different patterns/beams you have to vary the speed of the motors, so a pot is highly recomended. and depending on what volyage you are feeding the mirrors you will need to slow them down as at high speeds they will vibrate themselves into peices.
To reverse the motors you must have DPDT (double pole double throw) Switches.
Reversing the direction in continuous mode, requires a double battery ..... for the better regulation, instead, a simple voltage regulator (also just one made with an LM317) is the better solution, and a double deviator (DPDT) switch can be used for reverse the direction.