I don't do frame research. I just read it in my spare time.
And I don't know what frame you can slide out of F-tilt. Doesn't matter much to me anyway. F-tilt is one of those moves that is safe unless your opponent is buffering dumb **** during shieldstun, in which case it's possible for them to shieldgrab your foot if you're close enough.
And Xero, you were sliding backwards with those F-tilts right? Yeah that F-tilt slide. Honestly the best use for it is gimmicky baiting. Jab sliding, I don't know a thing about (unless it was in fact the F-tilt slide), but I can't imagine my response is going to be much different.
But as I recall, if you use a nametag, the way you can TIME the F-tilt slide, watch your nametag and iirc it dips down and then comes back up. Like, right at the moment it is coming back up, that is when you can slide. I think someone else pointed out that it has to do with when Mario is drawing back his foot or something.