Congrats on adding your second little girl!!
How exciting for you! We would love to see pictures!
I added Mikki to my family about 3 months ago. Rosie is not aggressive either but we found out that she CAN be! Mikki was 1 yr old and Rosie was 4 yr old. Since Rosie was here first, I always tend to her first. I feed her first, bath her first, pick her up first, etc. She is used to my undivided attention where Mikki was not. It seems to have worked for us.
I will warn you that with two females I see "cat fights" between the two of them. I understand they do this until they figure out who is dominate. Ours has slowed down considerably but we still have them. I think in time they will pretty much stop. You may not have as big an issue since Bella is so little.
If I can answer any questions at all for you feel free to pm me! I asked lots when I first got Mikki!
Good luck and congrats to you!!!!