If you think you have found a tumour, get her to the vet as soon as you can. You could have spotted it in its early stages, and as you can see it, that makes it fairly simple to remove. Surgery can seem a bit drastic for little rodents, but rats generally bear up quite well and any tumour is life-threatening so it's best to nip it in the bud before it progresses.

As for keeping them apart, I would keep them in separate cages facing each other for about a week. Make sure the cages are close enough for them to see each other, but not so close that they can stick their paws out and touch one another. After that week or so, under your supervision, move the cages together so they touch. Watch them all the time whilst the cages are together, and if there's any sign of aggression separate them again. Once you are satisfied that they know each other, let them into the same cage, but watch them for the first hour as they get to know each other, and check up on them as often as you can in the first week. Hopefully that'll do the trick.

I look forward to seeing pics of this new rattie!