UPM 1000, can metrics exceed that?

Can you tell me if it is legal to have your UPMs set to 1000 but to input larger metrics into the ascender and descender values to accommodate swashes, etc in Microsoft programs?
For example, I have a font that needs to have an ascender of 1200 and desc. of -425 so that the swashes don’t get cut off in MS word, but I want to keep the UPMs at 1000 so that I don’t have any compatibility issues. Is that correct and legal??
Note - I just changed the metric values - did not do custom WinAscent etc.

That is fine but you may run into issues nonetheless. Don’t forget to test in MS Word. If too much gets cut off, set the UPM to 1500 and add a parameter Scale to UPM 1000 in File > Font Info > Instances.

Related question: Are the metrics parameters only for proportion control? I have been advised to keep my cap height around 700. If I make all the metrics parameters proportionately higher, will my 12pt typeface be significantly larger than, say, 12pt Helvetica?

I don’t know your helvetica’s proportions, but most cap heights are between 60 and 70% of the UPM. But you can try and compare yourself. Make sure you avoid font cache problems:

Do you think it is better for the functionality of the font to enter custom parameters for the winAscent, etc? Or is it ok to just extend the ascender and descender values? I’ve looked at some popular fonts, and it seems people do it both ways. What are your thoughts?

Read the tutorial on Vertical Metrics, and see if it applies to your font (family). If Ascender and Descender stay the same throughout your family, the auto calculation may be just fine. Test and fix what is broken.