Well, IMO, the front 3 speakers are the most important, and to get a great presentation, that usually means matching them (same manufacturer/line of speakers). I'd look at a center than you like and then you can eventually buy matching mains too, or you could buy a matching front stage and use your current mains as surrounds (even if they are towers).
I'd probably add surrounds and a sub, then center. You can get by without the center for a while. Though it does produce quite a bit of the sound for movies, by setting it to "phantom" you can still hear everything from the front speakers. Some people prefer this, I do not.
How big is your room?
Hard to say what percentage, because it depends on your budget and how nice of a system you are looking to build. If you had $500, I'd say about half for the sub, then 30% on the center, 20% on the surrounds.