Can I put a newer stainless slide on an older 220 frame that originally came with a carbon steel slide? Would it be safe? Would it even fit?

Yes, I have done just this. I didn't have any problems. Mind you this was 94 vintage P220 & the stainless slide was from 06 time frame.

If the slide fits and functions properly there’s no reason it would be unsafe.

The one piece stainless slide is actually better for the aluminum frame because it doesn’t have a separate breechblock that must be held tightly to prevent the slide from flexing and putting additional stress on the frame. (That’s why the breechblock retaining pins in older slides should be replaced at least every 5000 rounds.)

Thanks to both of you. Now to find one.