Since there seems to be a lot of confusion as to what you are intending to do, MY reading of what you were intending (perhaps incorrect) is that you were going to keep your two good-quality existing speakers as front and left mains, and were only looking to buy some lower-cost "temporary" speakers for the surrounds, until you could afford something better.
If that is the case, then I'd stick with your current front speakers, set the center channel setting of your receiver to "None", so that you are operating in "phantom" center channel mode, get two speakers for surrounds at BB, assuming that is the best deal, and set them both to "small".
You don't want to spend money on a center channel speaker until you are set on your long-term solution for the front mains, so that you can timbre-match the center to the mains. Then you can upgrade the fronts and get a new center at the same time. You can either get a new "matching" set of surrounds at that time, or (depending on what kind of speakers they are, size, etc.) consider moving your current mains to new duty as the surrounds. (then you can decide if the cheap surrounds you bought today are simply "spare" speakers, or else maybe you use them as back-surrounds in a 6.1 or 7.1 set-up.
As someone else said - fewer good speakers is usually better sound than more, cheaper speakers.
In the short run, you only have a 4-speaker set-up to start, but you've minimized the cash outlay until you are ready for the "final" speaker purchase (yeah, right!)