Why don't you plant them in big tubs? They will stay dwarfed,can be moved around and you could take them with you of you move. I have a liquidambar that has been growing in a tub for 20 years, not bonsai'd just a normal big patio tub and it is still only about 4 ft high. I have done the same with parrotia persica too.
Be aware that Robinias have been dying from an unknown cause over the last few years, they are an expensive investment but it also might be upsetting for you if a special tree dies.