Fruit tree and nitrogen fixing trees

I want to plant a fruit tree on a sandy hill facing south, I have figured out that in order to give it a good start it could be accompanied by Elaeagnus angustifolia, Robinia pseudoacacia, Sophora japonica and Caragana arborescens (I have an access to these). Is it a good idea?

If I plan for, lets say, dwarf apple tree, what is the best way to use these species? First three can grow pretty big. In case of Caragana, it shouldn't be a problem, it tolerates pruning very well. Currently, the apple tree has a size of 3 feet, those four possible fixers have a size of 1-2 feet, but I'm afraid that they might outgrow apple tree soon.

If this idea makes any sense, how to space these plants?

As to preparing the soil, would burying some wood under such tree help to hold some water?

After planting, I plan to mulch with fresh wood chips, and plant other companion plants in small pockets of soil, makes sense?

Since it's a hillside and it's sandy, I'd think swaling would be very beneficial, and would create ideal niches for all the trees. If the nurse trees get too big, just coppice them and use the tops for browse, chop and drop, and larger pieces wherever you use wood....burning, hugels, etc. I would think twice about the black locust....it can spread vigorously by root sprouts, especially when coppiced, and the sprouts are quite thorny.

Many thanks for your reply. In case of swales, do you plant trees downhill from a berm or in a swale itself?

I've heard of both. I think it might depend on the soil....if there is any clay content at all, leading to a potential for water pooling in the swales for anything longer than a day or so, then one would not want to plant most trees down in there, but rather on the downhill berm and below. In a complete sand soil, planting directly in the swale might be the best spot......

Unfortunately no clay at all, whole hill is sand, people were mining sand here in the past, when they needed it for construction.