OOOH, this is a good one because, while I DO have MLP characters I associate with them, I just realized none of them are part of the mane 6. So I'll give you both, their usual association and the closest mane 6.

KIMBLE: Rainbow Dash. Sorry he gets one of the coolest and arguably doesn't deserve her but he'd think she's cool. He's made fun of Eddie for liking MLP and playing with pony toys but he'd think that one's cool.
Otherwise, I ussually associate him with Starlight Glimmer. They're trying to do better but doing horrible jobs at it... In my MLP AU I like to think he spent a while at her village, since he has no cutie mark at the time he felt like he'd fit right in.

SECOND: Fluttershy. Might seem like an odd choice, but specifically baby Second reminds me of her. Here's an image I did tracing over her because I think they'd have the same bodytype as kids - Tall and lanky. Otherwise, I tend to relate him to Princess Luna.

PETER: Pinkie Pie. He'd probably be embarassed about it, because she's pink and that's for girls, but he's a little kid. He'd learn that it's fine. And she's silly. He's a little silly! Otherwise the answer is Spike, 100%. He's a little dragon in MLP AU because of it.

ABEL: Out of the mane 6, Rarity. Definitely. High class fashionista. ...And well, if Second is Princess Luna, they're Princess Celestia.

COSMO: Cosmo likes early Twilight Sparkle. Before she became Princess Twilight. Cosmo cried when that happened. Otherwise, Cosmo's favorite MLP character is Cozy Glow and little Cosmo didn't understand why she was seen as a villain. It is very angry about Cozy Glow's treatment. She did nothing wrong but be neurodivergent and a minor.

STANFORD: Stanford is easy, he likes Applejack. He likes Horses and Cowboys and When People Are Honest so in this case both line up perfectly.