Looks like a good plan. The P gayi is a more open, airy plant so it should do fine in front of the Ludwigia. If it gets too tall it's very easy to trim. When it's happy it will send out runners everywhere, so you might want to consider a containment system for it.
I noticed that your background plantings are all about the same size. I did that in a tank and changed it later so some were bigger and others smaller - it looked a bit too regulated when the groups were all the same size. I realize this is just a general sketch, but it's something to consider when you're planting.