I have heard some apparently mismatched center/main set-ups that sound very good in spite of speaker design differences. If you do not watch movies that much and are not terribly critical of the sound coming out of your center channel you could get away with any number of different centers. If you can demo the speaker you intend to buy before you buy it I would recomend that. More often than not speakers sold as center channel speakers are over priced and low in performance in comparrison to its bookshelf counterparts. Single bookshelf speakers are sometimes available from dealers who don't mind splitting a pair but occasionaly available on A'gon for next to nothing.
Good luck.
Good luck.