Sand-blasting cabinets?

Stack all the parts in the cabinet in a pile if you can. The "blow by" will do some of the blasting for you on the parts you aren't working on just then, and it'll save time and wear/tear on your compressor.

Use lots of duct tape on the machined area of the spindles -- you don't want to blast those. You can use rubbing alcohol to remove the tape goo and then use some crocus cloth on the spindles to really clean and shine them up without removing any metal material.

Either of your medias will be fine for the aluminum dust shields. Glass bead is used if you want to achieve a "soft" finish on your metal. Since a soft finish is the last thing I need for my powders to adhere, I use Black Beauty slag for almost everything.

Can't help you out with the tip question. I only use one size. :-D

Have fun and be careful!