As GroundedSpirit said, you will probably receive a few responses from people telling you to slow down. I, however, am not one of those people. That is another traditional, conditioned mindset that people have to grow away from. Who's to say how fast is too fast for 'you' and 'your family'? What I would say is...always proceed with caution. I am of the belief that we do not conduct our lives in the way others see fit or what others have adopted as normal processes. You conduct your life based on what works for your and your loved ones. That's why we are mature (hopefully), growing, individuals and if the current pace is comfortable for all parties involved...you should continue with that pace and, hopefully, the communication is vast, open, and clear enough where you guys have no problem discussing all things, small and large, along the way.
Discussing your future desires and throwing ideas around is beneficial so that everyone can start pondering if the desires are the same, etc. There's nothing wrong with being sure of yourself and what you want. It cuts out a lot of indecisiveness and fumbling through relationships. I'm sure things will grow and some desires will change along the way but the foundation is laid which is what is important.
If it makes you feel more comfortable, and you guys decided to move the third party in, you should approach it more like 'helping a friend in an unstable situation out' because she obviously is going to need a place to stay. You guys can discuss allowing her to stay currently while she continues to search for an affordable place. If the living situations flows naturally then you guys can all decide whether or not it is necessary for her to continue to browse for her own place at a later date. Sometimes, no matter how great people are together relationshipwise, they may not mesh together well when it comes to living together so that may turn out to be a growing process.
All relationships are trial and error but as long as everyone involved is interested in putting forth a great deal of effort to learn, compromise, grow, understand, and unite everything can work out.
Sorry for the length.