This reminds me of the good old days of when I was doing GunPla modification on my modes. It almost make me want to do modeling again.

5830941 Which Gunpla do you think The mane six & Spike would build?

5830941
So why aren't you doing Gunpla like you're supposed to, then?

6565497
Yo, sorry this somehow slipped my mind all this time. As to answer your question, I think there are several mobiel suits that could fit them from grunts, ace to OP unit, but I like more then just your standard gundam hero units so I will try to widen the spectrum.
Twilight: Hi-Nu Gundam gundam practly powered by magic, why not
Spike: RB-79 Ball Still the side kick after all
Rarity: Qubely, it's feminine, elegant, and extreme accuracy with controlling it's bits, it's fits here like a glove.
Pinkie Pie: Char's Zaku 2 or Sazabi, probably him from behind with a crowbar when he wasn't looking and there is no way she doesn't have new type powers.
FLuttershy: Rx-121 Hazel Gundam it's the bunny gundam, like Angle, why wouldn't she want to pilot it.
Apple Jack: MSM-01 Serpent Custom, strong powerful grunt unit that it specs should stand it's own to a gundam
Rainbow Dash: Gundam Kyrios, it's fast and it fly, what isn't there for her to like.
Celestia: the RX-78 GP03S Starmen/Orchard could you imagine Celestia with a chariot/jousting mouth not wanting that
Discord: Turn X, the fact that it can detached and reassemble it fits him too much his personality and the sheer power of it sending humanity back to the stone age is not more something you want to triflyl with then with the lord of Chaos himself.
Luna: Double X Gundam, it used the moon to power it so there is no way she wouldn't want she wouldn't use it.
Big Mac: MSJ-06II-ATieren Ground Type it's bit it's ugly but get's the job done
Shining Armor: PLAN303E Deep Striker
Cadance: MSN-06S Sinanju it's pink like her why not
7492643
It's a complicated story, but life got very busy, and I preferred to focus on my art then on modeling, thought I would like to get back to it, I still have a number of them to build including my perfect grade RX-78-GP01Fb that I would like to get back to one day. BUt I don't know all the theory in modeling way before Youbube was even a thing.

7492853
I've not worked up to Perfect Grade. But unless you can't follow picture instructions, it's really just time you need. Don't think you can't, or you'll never know the satisfaction of seeing a finished model on your shelf. Especially a Perfect Grade.

Oh and reading another answer. The Sinanju is red, with gold trim. Not sure how you got pink. Unless I totally missed it in Unicorn 😲. And I think the Unicorn fits Twilight better than the Hi Nu.

But that's just me.

7492853 It's cool don't worry about it and great choices

I've not worked up to Perfect Grade. But unless you can't follow picture instructions, it's really just time you need. Don't think you can't, or you'll never know the satisfaction of seeing a finished model on your shelf. Especially a Perfect Grade.

Dude, I had built over 85 Bandai mecha models 20 Master Grades(all painted) 10 1/100 HG, 25 1/144 high Grade and not sure how many others excluding those old bootleg Evangalions ones from unit 0, 0a, 1, 2, 3, 4, and MP. when I stopped I was using pudy, epoxy putty, scratch build parts with platicar sheets, paint primer, masking, coating waterside and even using airbrushed them, all the works. Just simply assembling a perfect grade is a cakewalk form me, I barely need a instruction manual for it, but I just as the grade said, I wanted it to be perfect so I waited.
At the time my interest has shifted to Warhammer 40000 and there were a LOT of shit happening to me at the time and I didn't had the installation or place to do the work I wanted without constantly breathing in toxic fumes from all the material that I used, and my work process was a lot slower back then.