I would think that it would be better and less complicated if you didn't try to bring your fiancee's daughter at the same time. It would be different if you were married and were applying for a spouse visa. If you arranged to marry very soon after her arrival and then apply for a visa for her daughter after your married, she could be here within 6 months I would think.
In the UK children's welfare is looked upon as the responsibility of the state and the ECO's who deal with your fiancee's and her daughter's visa applications may worry about what will happen with regard to her daughter if you don't actually get married within the six month period, or if, god forbid, you split up. Try to see it the way the pen pushers might see it.
Iain.