You can use different speakers in different positions, but due to the fact that all speakers are imperfect, they will not be "voice matched" very easily if they are not the same series in the same brand. Perfect "voice matching" only occurs if identical speakers are used. This matters more for the front three than for the surround, but I have gone to identical speakers all around and will never go back to different speakers in different positions (excluding the subwoofers, of course). The reasons why the front speakers matching make the most difference are because of the fact that people hear better in front of them than behind them, and also because of the type of sound that is typically placed in the surround channels.
Normally, it is speakers that make the vast majority of the difference in sound, so typically one should focus on getting them as good as one can. If you are on a tight budget, you might want to focus just on the front three speakers and get them as good as you can afford.
Now, just because I want to use identical speakers all around, that does not mean that everyone else feels that way, so you might be okay with different bands in the different positions. With that in mind, you might want to try a pair of speakers for the right front and left and see how you like your current center speaker with them, though be prepared to buy something that matches the front right and left speaker if you don't like your current speaker with them.
If you want specific advice from people, tell us how much money you want to spend. There is no point in recommending speakers that cost $1500/pair if you only have $500 to spend.
