Now why did Rinoa instantly like Squall?
I think the writers put her in that position, intentionally. They weren't that dumb
That's why something is going on, putting some doubts on the "o no, finger sign is just a greeting sign" procedure.

They're not complete strangers, and this may very well be a "first sight" thing (which really does happen).
In Squall's mind, they were, the dancing party is where they met for the first time.

There are other doubts, as well. Think about why you see the splitting image of Ulti AFTER you see the finger sign.

I'm tired of people saying O NO THEY WANT TO DANCE, THAT KIND OF SIGN. I already know that, I know it happens in parties a lot, BUT there are so many doubts/clues that may tell something else.. that the sign wasn't exactly a greeting sign/nor star