I would probably put them in the 75g with the rainbows and bettas. The 9g would be too small for a pair of plecos. Even though zebra don't get very big they are still plecos (waste producers) so you'd be having to do lots of WC's if you used the 9g to keep good quality. And the cory cats in the 32g would compete for food with the plecos. So in the 75 the rainbows should leave the plecos alone, the bettas "should" leave them alone as well but keep an eye on them for a while to make sure.