Is it unfair for me to start dating someone when I've still got slight feelings for someone else?
No, go for it.
It'll help you put things into perspective.
However, if you get to the exclusivity phase where you (or at least they) begin to become emotionally attached, end it if you think you're only one foot in
Everybody's still got slight feelings for someone else. But I think you mean more than slight.
>guy is amazing and we get along perfectly
>he's really into me and i'm into him too
>he wants to start actually dating instead of just being a thing
>i really like him but i still have slight feeling for a past oneitis
>i know i'm not going to find a guy who's into me as much as him though
>i know i have no chance with oneitis anyways, and even if i somehow started dating him we probably wouldn't last
To put it square, you're kinda fucked.
Either you don't take the chance, and you feel horrible for it.
Or you do and you end up breaking that guy's heart.
The only good end is if you fall for him, but how likely is that?
I'd say date and deal with the consequences.
You're likely to feel shittier than you do now, but it's the only course of action that has the possibility of having a good end