A long time ago I did a fusion of Sonic and Master Chief, I went back and did a little joke with it. I wanted to make fun of the narrator from halo and unreal tournament so I did this. Its not as funny as my other sonic shorts, but its funny.
I never truly pictured Sonic as a Spartan. The reason why would be simply the fact that he may be too overpowered in such a position. Sure, he would be insanely fast especially given that the suit would amplify his own abilities, but for me, it was also a deal for character development. Sonic would have needed to go through augmentations and would be handed a weapon. The way I designed it, I don't think he would have accepted to do either, possibly on the basis that he would be forced to kill and to essentially go through the enhancement process which he could reasonably die from.
Actually, i didn't make any of the main cast Spartans because they seemed to get too much spotlight as was. I tended to take a different path where regular Mobians took up the offer to become Spartans. From their perspective, why would they not jump at the chance to become supermen? People like them envy Sonic as their idol, and given the opportunity, they would have taken the chance to become Spartans. Though applicants would be active military service members anyway, who had combat experience. You wouldn't just grab someone off the street and lock them in the suit.
It may sound like I'm overthinking this, but I have literally built a whole world around this, and the question of who gets to be a Spartan or not was a question that popped up very early, and the answer was: on the main cast? Nobody.
Master Cheif: WTF, a hedgehog spartan?! SonicIE!!! *shoots a spartan laser* Master heif: AHHHHHHHHHHHhhhhhh *Dies* Sonic: i guess i'm the master cheif now... *shotgun blast* Sonic: AWWWwww.... Me: Ha ha! i pwned you stupid noob! LOLOLOLOLOLOLOLOLOLOLOLOLOL!!!!!11!!!!!! *Gets shot by a sniper* Me: SHIT!.... Me: