It's my pleasure! We've all started where you are right now and still continue learning with each passing day! That's what I love about this hobby
Lots of swimming space is good! The tangs should be happy.
Since this is your first saltwater tank and I want you to succeed, I would nix the powder blue simply because they are super sensitive to EVERYTHING!
If you want multiple tangs, I would go with the yellow and maybe a bristletooth tang (kole, tomini, two spot, or squaretail). Avoid the chevron tang as they're one of the larger bristletooth species. It's hard to mix tangs that are similar in shape, like the yellow and sailfin. Also, 125 is still borderline for a yellow tang alone so adding a sailfin, which grows quite large, is pushing it.
The best suggestion I can provide right now is to add your fish slowly. Add one or two fish at a time (obviously add both clowns from the pair simultaneously). Get your cleanup crew in there (cleaner shrimps, astrea snails, trochus snails, nassarius snails, emerald crabs, maybe some hermits) and be sure to follow acclimation procedures because you can't just adjust these creatures to temperature and then throw them into your tank. Test the water regularly and if everything seems stable from week to week, then I would say it's safe to add more fish.
If you have more questions, feel free to ask them here or to send me (and others you trust) a private message. We're here to help
