I've got two early 1980s roadbikes I'm retrofitting into hybrids-- a Schwinn Supersport and a Raleigh Competition GS.

The Schwinn was tricked up w Suntour Superbe Pro derailleurs & hubs w 7 speed 12- 24 ultra freewheel; I want to make it a little easier on the hills, but can't find a 7 speed ultra XX-28 freewheel (lots of 30s & 34s out there, but don't think my derailleur can handle them). Question is: can I add an extra cog to a 12-28 New Winner ultra, or should I just stick w six speeds?

I'm upgrading the raleigh w dura ace hubs and a crane GS; freewheel is currently a dura ace 12-24; question here is, can I switch over to a Suntour New Winner 7 speed 13-30 ultra freewheel, and will that work with a NIB Dura ace ultraglide chain? If not, what's the best way to get a compatible freewheel in the 12/13 - 28/30 range.

shifters in both cases will be vintage suntour barcons mounted on Scott AT2-LF bars; will have ritchey brake levers w dura ace brakes on Raleigh, Royal Gran Compe 500gs on the Schwinn

thanks for any thoughts

The Shimano 11-28 7-speed should work fine, no?
https://sheldonbrown.com/harris/freewheels.html#7

1) 7 speed Sun Tour Ultra freewheels are a little thin on the ground and tend to be pricey. I'd try standard spaced freewheels and a couple of spacers on your axle.
2) The Shimano 11-28 is a nice item; I'm putting one on the Dave Scott Centurion I'm re-habing. For a 13-30, try a Sun Race freewheel. I've run one on a bike for two years now and it runs fine.
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thanks for the suggestion/link. looks promising -- but can the newer hyperglide splined freewheels be used to replace an older, unsplined uniglide one? (and will a uniglide chain work? I've got a nice NIB one)

If I understand your question right, you can't replace a thread-on freewheel with a splined freehub. I don't think going from a freewheel with uniglide cogs to hyperglide ones will make any difference. Unsure about the chain.