Yep, I would stick with Polk since you already have the center. However, if you find some other speakers that you like, you can always make due with the Polk center until you can get a new matching center. Just more money that way. The key is not to compromise. If you don't like the Polk speakers, you should look elsewhere and get the center later.
BTW, if you have the sub already why not get a set standmounts instead. No real need for floorstanders. Might save enough money to include a new center.
For starters you might look at,
Paradigm
Energy
Boston
Polk
Infinity
Axiom
Rocket
Mirage
B&W
Athena
Cambridge Sound Works
Ascend
PSB
Shinerman