If you have enough water and want a very small tree, it could be OK for peaches, but I think Nectarines are a bit less vigourous and I’d avoid them on Citation in the future.
15 feet seems like a lot to me, but I suppose if you have the space it would be good. You’d have plenty of space to work around them then and avoid them growing together. Even at rentals where I wanted to space things out, I only used 10 feet…at most.
I’m not sure of the ideal shape for plums. I’ve been cutting them back to whatever fits in the space, mostly central leader when the tree cooperates. I think if you have enough space, open center would be good.
The So I planted in 2011 has been fruiting well for the last 3+ years. But, 95% of the fruit is on the South side of the tree, which I think demonstrates the need for sun.