So if rangers need not have pre-requisits for their combat style, is it legal for a non ifrit race ranger to use the elemental style (scorching weapon) which has the ifrit race as a pre-req?
by RAW i want to say yes ... but it feels questionable.
Yes.
Being a particular race is a prerequisite like all others, and rangers cheat all prerequisites for their bonus feats.
Absolutely yes. Just keep in mind that if you aren't an ifrit, you need the combat style feats to take the follow-ups (Inner Flame and Blazing Aura) at all. Of course, this isn't a bad thing; the ranger class lets you buy into those early.