I only have one dog, but I do have some experience with this.
I got a second rabbit, Hannah, with the intention of bonding her to my first, Otto. From the first day Hannah was kind of crazy (digging, shredding, chewing) and bonding was not easy. Otto was polite, but she chased him, bit him, tried to hump him, and generally terrified him. I very seriously considered sending her back for a couple weeks. I stuck it out and fully bonded them, so then I couldn't send her back, but I still wasn't sure. It wasn't until we owned her for about a year that I felt like she really fit in and I was able to see all of her great qualities. I think this is all really normal.
As far as bossy dogs go, Watson's best dog friend is a bossy ACD. It definitely requires management when they're together - no high value chews unless they are closely supervised, and we have to make sure the ACD isn't controlling who Watson can get attention from. But really, they do very well together because Watson is easy going and they have worked out their relationship. It *can* work quite well, and just requires that you let the bossy dog know that her bossing isn't necessary or tolerated. I think this type of behavior is something that is definitely worse at the beginning, not something that gets worse over time (unless you mamage it the completely wrong way, which I don't think you will).
Good luck. I think what you're feeling is totally normal, but that doesn't make it easier. I know that awful feeling of "oh my god, what have I done?!"