The differences: PG 1050 cassettes are a bit heavier due to having smaller holes cut in them, also the surface treatment is less hard-wearing than the better cassettes.
As far as the differences in the PG1070, the MTB versions have a much more open design to imporove mud clearance in off-road use. The road cassettes offer a wider choice of gear range options.
You will be able to use a SLX GS (medium cage) derailleur with an 11-36 cassette, but NOT with a triple-ring crank. If you want to use a triple, then you need a SGS (long cage) RD.
If you need one, I happen to have a SLX GS RD spare at home that is no longer needed. It was in my HPV for a while, custom-modified to a SS (short cage) format, and while it fixed some problems it caused others. Ultimately my engineering proved to lack durability, and the derailleur has since been returned to its original configuration.
Just ride the bloody thing and enjoy yourself!