I know that I can't take advantage of the new features of v9 such as conditional formatting, etc. in this situation but would like to make the most of script grouping, better script debugger, etc.

Is this a safe practice?

Thanks for you advice.

"Is this a safe practice?"

Yes, as long as you eat less fat and exercise regularly.

I'm also re-formatting the layouts to take advantage of the resizing features. They still look normal in 8.5 but do their thing in 9.0.

I'm also going through and putting in a "check the client application version" routine into the startup script of every file, which closes the file if the wrong FMP client version is being used. I'm setting it to 8.5 if it uses script variables (or any other 8.5 feature) or 8.0 as a minimum, because *nobody* should be using 7.0 any more. (Ideally nobody shoud be using 8.0 either because v1 and v2 had some horrible nasty bugs.)