Others might know better, but I don't think I'd mix anything with UB, other than noogling and maybe something like collagen, msm, and/or spirulina. Most other breast enhancing herbs/products are estrogen based, and UB is progesterone based, so I'm not sure if they'd work together or against each other. UB is supposed to be pretty strong by itself, not sure if it's as strong as BB because they're processed slightly different with different ingredients. And don't forget that UB recommends intaking a lot of protein in the form of protein shakes in order for their product to work properly. Also, keep track of your temperature for a while before you start and make sure you maintain a body temperature of at least 98.2, otherwise you'll have to take another supplement for a while first to raise and maintain your body temperature high enough, I'd have to because mine is usually in the mid to upper 97 range.
Oh, I almost forgot one other thing you could take that is progesterone based, wild yam. There are various pills you can take and/or creams you can use, I use this one...
http://www.luckyvitamin.com/p-14247-good-n-natural-14-wild-yam-cream-with-aloe-and-vitamin-e-4-oz
It has a higher wild yam concentration than a lot of the other more expensive creams.