Well, if you want to get extra technical, it's possible to put the models on those characters. BUT:
(Text Wall Version)
1. If you wanted them to have the original animations, first you'd have to redo all the animations from scratch while having to get past the filesize limit on their animation stuff. Which is a lot of work, considering their limit is so small.
2. You wouldn't be able to port their original movesets. Marth's would be doable, but you'd have to redo it from scratch. Snake's would be possible in theory, but you'd have to port and add so many external models that it would take up too much space. Olimar isn't even plausible. Olimar has A.I. that follow him around which can't be ported. And on top of that, you'd have to fit all of this into their tiny filesize limit AGAIN.
3. You'd have to make them new movesets, which is incredibly hard on Pokemon Trainer. The Pokemon don't work with the newest version of Project Smash Attacks. They only work with the older version, which is prone to corrupting the Pokemon moveset. On top of that, I'm not even sure they work with BrawlBox's moveset feature. And once again, you'd have to deal with the filesize limit for PSA.
On top of that, the filesize would be huge. It would be three separate high-quality models along with three separate professional PSAs and the separate sets of fluid animations. Each one of those numbers up there could break the limit by themselves. Put them all together and you'd overtake the filesize limit like a Mustang on a VW Beetle.
TL;DR
You'd have to be an expert in every field from 3D Modeling to Porting to PSA to even attempt it, and it'd STILL be near impossible.
HOWEVER, if you just wanted to put different models on the Pokes and make a moveset, you could still do that. They'd just have to have around the same filesize as the original.
