What the previous few posters have said is absolutely true.
My female, Maddie, is sooooooooooooo playful and silly and rambunctious. She plays plays plays with any and every dog she sees. She has a great time at the dog park, and generally gets along with any dog she meets. I get together with another person on these boards (who has 2 females) and we meet up and let our dogs play. They have a great time! No dominance issues whatsoever.
You would think that we wouldn't have any issues here, but Maddie just CANNOT coexist with another female. This is the reason her original owner gave her up for adoption (severe female/female fighting). Something happens to my sweet girl when another dog is brought into our household to stay.
When we brought Lennox (our 2 year old Foster Boy Boxer) into our house, we actually had the same fighting issues, despite the fact he was a male. It turns out Lennox is dominant as well! It took us months of working with both dogs just to get them to coexist somewhat peacefully. Ultimately, we couldn't keep the two of them and Lennox now belongs to my parents, who fell in love with him (and live here in the same City). The pooches are much happier being an "only" dog -- but ironically, they love to see each other and play.
My (long-winded) point, is that even if your dog *seems* playful and not very dominant, there is NO gurantee that she will get along with another dog in her household. I still feel that opposite-sex pairings work much better, as males tend to "fight, forget, forgive and move-on". Females will ALWAYS remember and hold grudges...
Good luck