Yes, a Unitec can interface to older Softgloss machines, but how it is installed will vary slightly depending on vintage of the machine.
If your machine is old enough where you have separate inputs for the various wash options (wheel scrub, dry, 1 pass wash, wax, etc.), then you will need to install a Wash Combination Selector to work properly.
If your machine is slightly newer and you have inputs for different wash packages (package 1, package 2, etc.), then it wouldn't require the wash combination selector, and the outputs from the Unitec can directly be wired to those inputs. If this is the case, someone will need to go into the program and edit the wash program bits to setup the wash packages using an HPP (technicians will carry these).
In either case, there will be a setup question (Other Arming?) that will need to be changed to "YES".