i don't think you need a larger master cylinder for just adding rear discs...i did this and never had a problem with brake fluid starvation in the MC...not much more fluid will be required to actuate the rears discs over the rear drums...however if you upgrade to larger calipers especially 2 or 4 pistons you will definitely need to do this

while you have the rear LCA from the teg i'd replace all the bushings with polyurethane units from Energy Suspension or Prothane, etc. since it is removed...i'd also get the bearings repacked as you can just bring the lca into the shop...this just reduces any future potential headaches...and if the parts car is old and rusted bad i'd consider replacing the rotors, pads and calipers just as an added measure of precaution

good luck and the site listed above is a good diy companion