Havoc - the biggest factory rim in the Rally IIs Pontiac made were the 15 x 7s. 15x8 Honycomb and Snowflake wheels came along later. You should be okay with either set so long as the bolt patter is 5 x 4.75, which is the same as the GTO/Firebird. I checked to make sure the X-body platform carried the same bolt pattern. So you have your choice of rims, although if the chrome rings are in good shape, the 14s might be the better buy since the wheels are already painted (albeit used). The 15s you have to buy everthing else, paint (argent silver and charcoal), center caps, and trim rings (if you want them).
You might want to buy a fresh set of chrome lug nuts to go with these new wheels.
As for the question of rust, you want to buy some sort of rust converter, not a rust inhibitor. Naval jelly is one product, Eastwood has another. You want to sand off the large scale rust (and paint) and then treat the rest of the metal with the rust converter, cover it with a sell-etching primer, and then paint/clear it. Be sure to read all the instructions on proper surface preparation because some special prep maybe necessary between converter, primer, and paint.
Paint Over Rust (POR) products will encapulate the rusted metal and prevent air and moisture from getting to it. POR products are not generally recommended for things like exterior door panels or fenders that need to be painted to make the car look good. However they are great for chassis / suspension parts or inside doors, fender wells, etc. that will not be seen. That said, I know POR is coming out with a new line of products, so they may have some newer materials to work with too. I have a brake caliper kit from POR that I want to try when I get some spare time for my kids perennial baseball season.