I agree with @ztom
You could use Adara plum as in interstem for grafting peaches to be safe if choosing Prunus americana. I have some Adara wood if you need it.
Prunus americana will also naturally dwarf your trees which can be good. It picks up speed though and you’ll be pruning your trees annually and moderately-heavily your cultivars on P. americana.
A very knowledgable home horticulturalst member has many apricots directly on Prunus americana w/o problems near or beyond year 10. I have ‘Hoyt Montrose’ apricot grafted to americana and it looks pretty good after 2-3 years.
What else I can tell you about P. americana and overly vigorous cultivars grafted to it is that the unions are very out of proportion. My pluot ‘Flavor Queen’ has this issue. Most other hybrid stonefruits look fine. That Flavor Queen has exceptional vigor and it has shown so.
It never hurts to mound new-plantings, but I don’t think it to be a necessity.
You seem to me to know a lot about hort. in general with the few words posted. You know not to amend unless absolutely necessary, and that’s saying a lot.
I don’t recall the parameters for St. Julian. I once read about it and figured out from the group quickly that it’s more of a west coast rootstock. If I remember a Pacific Northwest rootstock. Probably a good choice for northern California where the redwoods are; maybe San Francisco since there’s a lot of water in the air there, too. And they have Redwoods. Long Island where you are I’d assume is very mild and moist too. Like I say, St. Julian has kind of taken off my radar and I didn’t read up upon to write this post.