Sally only exists in the comics, Amy has always been his mainline "girl" but Sonic doesn't actually give a shit because like you said Amy is a kid, Sonic isn't actually that much older. Maybe in ten years Sonic will care what Amy wants or he'll find his own girl since girls fawning over him is probably too easy for a guy like Sonic he'd go for someone unique.
Sally is in one of the major and popular cartoon series too and several novels. She is as much canon as Amy, literally, as even Amy probably has more contextual appearances in the comics than any game ever and her obsessive nature in the games is really just a diluted take on her far more reasonably portrayed infatuation with Sonic in the comics.
And Amy still isn't actually a love interest for him, that's why I said 'ship'. Because that's what it is. Her first appearance was in Sonic CD when she was a child (literally 8 or 10 years old, dude...). Since then, her infatuation has always been portrayed as a childish crush and joke, until she was aged-up (only ever explained as being due to a well-meaning but wasteful wish), where it continued to be a running gag more or less.
Correct me if I'm wrong (I'm sure there are a lot of recent offshoots I've missed now) but in
all the classical stuff the fandom is based on, Sonic never ever once reciprocates Amy's interest and in the few pieces of content where their relationship is ever explored (like the comics), he continuously turns her down full-stop.
So no, she has not 'always' been his mainline 'girl.' Still isn't, far as I am aware.
Edit:
And just to be perfectly clear, I love Amy as a character, even in her early days, and really like how her character developed and was increasingly included as a standard part of the video games crew along Knuckles, Shadow, and Rogue now too. I and a lot of people just dislike how they keep pushing this simping trait of hers that stopped being character-defining ages ago, especially since they clearly have no interest in actually putting them together.