Bob--
you want to use baking soda, not Borax, to raise your alkalinity. Borax raises pH. I personally would use the muriatic to lower the pH (which will drop the TA some) to where you want it, and then use baking soda to raise the TA back up to where you want it. However, I don't have a newly done plaster job. My advice would be to check with your contractor to see which he wants done first, so that you don't void the warranty on the plaster job.

Dan--
I would not ever mix chemicals before adding them to the pool, no matter what you're adding. However, once they are mixed in the water, they are fine. You don't ever want different forms of chlorine to come in contact with one another, without mixing with water first--they can result in explosive consequences. So--if you have trichlor pucks in a skimmer, you don't want to add anything else through the skimmer except for Borax. If you remove the pucks, you can dilute borax, baking soda, or granular CYA in water and put it in through the skimmer. When adding cal-hypo granules or acid, I would add them slowly into the return stream where the water returns to the pool from the filter. Same with bleach--the jets help distribute the chems.

Does this help answer your questions?