Welcome!
1. Just about any salt mix will be fine, some more fine than others. I think there was a thread in the Reefkeeping forum about what everyone is using. Edit: link to thread: http://www.austinreefclub.com/topic/12940-what-salt-and-why/page__p__89350__hl__salt__fromsearch__1#entry89350
2. No comment specific to SWC but I would buy slightly larger than you think you need. Your total volume is 75g(based on tank sizes only), I'd get something rated for 100-120g.
3. Short answer, yes. You will probably find that as you acquire corals you will wind up getting some that are attached to rock which will move over to your tank thereby increasing your total weight.
4. Yes, SPS can thrive under T5. You just need to get an appropriate setup, either fixture or DIY.