Would Cregan Stark have succeeded in destroying the green supporting great Houses like Baratheon, Hightower and Lannister?

[Mario Seddy]

We know that Cregan Stark wanted to destroy all the  Houses that supported the Green faction but would he have succeeded in destroying Great Houses like Baratheon, Lannister and Hightower 

[Ran]

Pretty sure that the answer is no.

[Lord Varys]

I guess if he had been able to convince the Velaryons, the Lads, and the Lords of the Vale to support him he could at least have take Storm's End. House Baratheon was down to women and a newborn infant, apparently, so it shouldn't have been that hard to attaint them and grant the Stormlords to a different house.

Oldtown would have been a harder nut to crack - for that he would have needed the support of the Tyrells, the Black houses of the Reach, and at least little to no resistance from the Green houses in the Reach. With Sam Tarly being married to Lord Lyonel Hightower at least the Tarlys - who were Black - would have stood with the Hightowers. Still, I guess, one could have convinced a lot of men to participate in a campaign against Oldtown in light of the plunder there.

But it is pretty much impossible that they could have taken/destroyed Casterly Rock.