Old bike compatibility with direct train bike trainer?
The trainer has a standard Shimano freehub body. It is not advertised, but it is entirely possible to install a 7-speed cassette and use spacers to compensate for the cassette width difference. For even lower number of cogs, you use cogs from a cassette and install the cogs with extra spacers to match the spacing on your current cassette or freewheel.

The other problem is axle width. Modern road bikes have 130mm real axle, and older ones 126mm. You have to either spread your frame a bit or ask Wahoo if the axle width can be adjusted.