I have a 2002 Specialized Allez triple that came new with Sora 8 speed shifters and a 9 speed Tiagra RD, with 8 speed cassette. It works fine. It also had a Sora front derailleur which broke, replaced by an Ultegra.
FWIW I also used Sora 8 speed shifters with a 7 speed cassette on another bike. It worked fine, just didn't have a gear for the last click.