I watched them all in order, and it didn't ruin the experience one bit for me.
I thought it was even better that way. Watching Index, then Railgun, then Index II, then Railgun S was the right order for me. Index does indeed spoil things for Railgun S, but it doesn't ruin the experience. For me, at least. Railgun was meant to focus on Misaka anyway. It intersects with Index at only one arc, which is the Sisters' Arc. But that's it. The timeline of the events might intersect, but that's the most that they overlap. At least, for now. Watching the Sisters' Arc in Index was nice, but after watching the Sisters' Arc in Railgun S too? My god. Everything was made clearer and better. I liked Misaka even more after S. She was amazing.
Meanwhile, started watching Kabukimonogatari.