I'm curious what problems you're running into with Engine Viewer 10.2.1a on Win10? I'm running ArcFM 10.2.1 on Windows 10 with no issues (shhh...don't tell SE tech support).
As for your question, I don't think you should have any problems with what you're thinking. Newer clients can typically read older data versions with no problem. Just don't expect much tech support help if something should pop up.
I'm getting a sandbox Win10 box set up for more in-depth testing (didn't know 10 was being pushed to some trucks), but the issues described include ...
- microscopic menu text (File, View, Tools, etc.)
- tooltip txt so large it wont fit in tooltip bounding box
- Locator results box unable to accommodate found features (no scrolling\resizing)
Maybe a graphical setting somewhere in Win10? A quick upgrade to a supported 10.2.1c might be the most efficient solution in a tight timeframe.
What hardware are these Windows 10 clients running on? I've had some weirdness from time to time with Responder resizing its text, but I'm on a 2-in-1 tablet style PC that plugs into a port replicator for dual monitors and that can confuse Win 10.
I haven't tried ArcEngine on Win 10 yet, so I can't say if there's something with that particular product that would not like Win 10. It almost sounds like it might be a video driver issue. If you're 10.2.1c test has the same issues, I'd look at the hardware itself.
Just to put a closing comment on this topic...
ArcFM Engine upgraded to 10.2.1c will read a 10.2.1a database without any problems. Sandbox laptop testing confirms no issues.
Original display issues turned out to be out to be a PICNIC error (Problem In Chair Not In Computer). User had changed a Win10 display setting called "“Change
size of text, apps and other items” to be set at 150%. Restoring this to 100% solved display issues.
I wondered about that setting, as I played with it a little bit when I first got Win 10 and experienced the port replicator issue. But I never quite experienced what you were describing.
Oh, and I prefer the PEBKAC acronym myself...