I've used 80's 6-speed 105 RD and dt shifters with a 9-speed 12-25 cassette and it worked just fine. The rear shifter moves a bit further back to get into 1st gear, but the individual shifts worked smooth.
Others on here have told me that using a 9-speed chain is ideal with 8-speed cassettes, and so when I did that on a different build I found it to be perfect.