Ponos,
I don't believe SA International has placed their demo version on any downloadable ftp site. However, any of the installation DVDs that they provide does have the option to run their software in demo mode. You may want to check with your local SAi dealer on obtaining a disc.
The latest version of Flexi Mac, 8.5, was written for the changes Macintosh made in Leopard... or OSX.5, but will work on 10.4.
I wouldn't rely on anything lower than that.