Well, frankly, a Gelgoog was actually a better suit than the original RX-78-2 by the end of the OYW. The RX only continued kicking ass because Amuro was Amuro, and the only truly powerful Newtype we saw during the OYW. Char was a bit of one, but he never really had much power. By the time he got his Gelgood (and certainly by the time he got his Zeong) Char had better mobile suits than Amuro did. Amuro was just the better pilot.

I'm not saying OYW suits will be able to take on all of the CCA era's mobile suits, especially not with a halfway decent pilot, but standard Gelgoogs were a decent threat as far along as ZZ. And the Earth Zeon remnants in Unicorn almost completely overran Torrington base using mobile suits that couldn't be any newer than 0083. Long story short, something at least as recent as Zeta or ZZ should be able to hold up in the CCA era just fine. I mean, the Sleeves in Unicorn used a lot of Z and ZZ era suits. 

Have you considered who the villains would be? Char's gone by this point, so Full Frontal?