This could get really long. First, start by looking at your own relationship with the current girl. Is she really willing and able to control her jealousy? How about you? Can you contol yours if this girl turns out to love your girl more then you? All of these things can happen.
Next, Look for a girl that is bi, not just straight or bi-curious. I've had a number of them come to me but were really interested in pushing the other girl out. Be prepared to always show the first girl the respect due if you should end up loving the second girl more then the first. Yes, it could happen. Be honest with yourslef and with her and talk about all this before you start.
Decide what kind of relationship you'll have. Are you Dom and your girl is the Alpha and the new girl is to serve her too, or are you Dom to both and neither has authority over the other. This needs to be defined and agreed to up front or you'll wish you'd never started this.
They should be about equal in desire to serve, A girl that is a switch will never work with a girl that is a total slave, unless the first is allowed to be the Alpha girl. If you want to have those wonderful three-somes, then they should be balanced in their need to serve you and themselves.
When there are 2 people, there is one relationship. When there are 3 people, there are 4 relationships and everyone has to be committed to make them all work. One weak link and the whole poly relationship will fail and it'll be really hard to pick up the pieces. It is constant work, but when it does, as mine does, it is so well worth the effort.
Similiar height, weight is nice, but I have one that is 36 and the other 53. One is 170 lbs and the other 250. One has children and the other raised them years ago. But, amazingly it is working and working well as they are both slaves and totally love me and love each other as sisters.
I hope this has helped some.