You can mix speakers if you want to, but it is usually best for them all to match. The front three matching each other matters more than the rears matching the front. The reason is, when a sound pans across the front, it should not change in tonal quality based on which speaker it is coming from. With the rear, it matters less both because soundtracks usually make less important use of them, and also because humans can hear in front of them better than they can hear behind them. Since you already have something that you want to use for the center, I would suggest buying a pair of speakers for the front right and left and try them with your center, but be prepared for the possibility that you will want to replace the center channel speaker so that it matches whatever you buy for the front right and left channels.
Ideally, you would use identical speakers in all positions (except subwoofer), but most people don't do that. I use identical speakers (not merely "voice matched") for all channels, and I will never go back to anything else, but some people don't feel it is that important. If I were on a very tight budget, I would not worry about the rear speakers matching the fronts.
For the subwoofer, I recommend that you wait until you can afford at least the bottom of the line SVS subwoofer before bothering with an upgrade from where you are. Their cylinder subs usually give the best value, but some people don't like their shape.
Ultimately, it is your subjective opinion about what is "good enough" that matters, along with your budget. The tighter the budget, the harder it will be to get great sound, but you can get pretty good sound for not a lot of money.
And you are focussing where you should; it is the speakers (including subwoofer) that affect the sound more than anything else. Also, if you buy great speakers, you do not need to upgrade them when new features come out, so putting your money into good speakers is doubly wise.
You should listen to as many speakers as you can stand to listen to in your local area, of as many types as you can find (e.g., ribbon, horn, dome, whatever), of as many different brands as you can find and stand to listen to. That way, you will get a good idea of the sorts of things you like, as well as what you don't like. When auditioning speakers, it is good to listen to a variety of types of music with which you are familiar. You might want to make a CDR of selections for this purpose to take with you to local audio shops to audition speakers.
Keep in mind when selecting speakers, you have a subwoofer for the deep bass, so focussing on upper bass and above is wisest. You don't need the main speakers to have incredibly deep bass when you have a subwoofer to take care of those frequencies. So spend your money on speakers where the money wasn't wasted on deep bass instead of better sound for the upper frequencies.
