Some people can't stand mismatched mains and center, others could care less as long as they have sound. The best sounding systems are all matched speakers, obviously, but the surrounds can be different from the mains and center. It is up to you whether or not you can live with mismatched sonic signatures or not. Your old speakers may work as surrounds as well, at least until you can buy some QS's, this would allow for the more critical LCR to be of the same timbre. It just gives a nicer/cleaner soundstage when the front 3 sound the same or as close as possible.
