I just assumed that because the trim rings fit, wheel covers would fit, too. Definitely would have to remove the center caps, though, which means removing the wheel from the car. So it would be a bit of a hassle. Also, the edge of the steel rim tends to show around the wheel cover, and it should be black instead of argent. But touching up the outer edge of the rim with black wouldn't hurt anything, since it would be behind the trim ring anyway. At least on my 14" rallies, the brushed trim ring sits higher off the rim than the standard wheel covers did on my steelies. But this probably varies a lot.
Oh, one practical factor: the reason I swapped to rally wheels is because the full wheel covers tended to pop off under hard cornering (as in the Bullitt chase scene). Trim rings don't because they aren't rigid in the middle — they can flex with the wheel. May or may not matter to you... I got tired of retrieving them from the ditch.