any science to mix and match speakers for HT?

Ardent two channel audiophile wants to create entry-level home theatre. I wanted to create 5.1 system using my 13 year old Thiel 1.5s as front R+L and wanted to know if anyone thinks it is very risky ( for poor final sound) to just buy any decent used mid level audiophile quality center and rear speakers? I already have a decent mid level velodyne sub. Thiel has a rather high end center that is above my budget so I was looking at others.
thanks for advice