Thats probably why they didnt use it.But one way it would work even with being ugly is if it was a hybrid design created by the crew of Theseus.Similar to how the Galactica crew created the custom designed black bird stealth fighter.
They didn't use it because it didn't fit the plot. It's probably a reject design for a plot line that never happened. Making up a way for the Theseus to have it would be forcing it in without much sense. When the Chief constructed the Blackbird on Galactica it was under a very specific set of circumstances - they were on the run, losing fighters with no way of replenishing them, so Chief tried to do something about it. Even then it was considered a miracle he was able to construct a ship that actually flies without killing the pilot, even so the ship could only ever be used for recon or sneak attack, couldn't replace the vipers in real combat.
Now, the plot around the Theseus happens during the fall of the Colonies. The crew would neither have cause or time to go design a new fighter. I'm pretty sure it's not standard procedure to go design a new fighter line aboard a battlestar to pass the time while patrolling. That's a bit like the crew of the USS Nimitz deciding to design a successor for the Super Hornet because they feel like it. It just doesn't make sense on any level. A fighter that looks like a mix of colonial and cylon tech would make even less sense considering that the colonials know next to nothing about Cylon technology at the time of the attack.
So no, we're not going to force it in just for the sake of having it, creating enormous plot holes in the process. Also, it's ugly, we don't like it, it hasn't appeared on the show, wasn't mentioned either, therefore it's not canon and can be ignored if we so choose. If anyone wants to mod it in once Diaspora is out, of course that's fine by us. But I think it's pretty safe to say this thing will never appear in an official Diaspora release.
I see the difference now.You should probably just go with it being an advanced or upgraded version.
Amazingly enough we did plan on making the advanced raider more.. advanced