Well awhile ago I came across this website talking about grafting multiple scions onto a single pereskiopsis. It seems like a good way to fit as many grafts as possible into a small space. Here's a link to the website
http://www.lapshin.org/cultivar/N12/articl-e.htmI've done this with mostly lophophoras so far, but have recently started trying with trichocereus bridgesii seedlings too. I've been getting a better success rate when using fatter seedlings. Its a little trickier than normal seedling grafts, but not much. After a couple tries you get the hang of it. I'd recommend trying a few normal seedling grafts first and reading New Wisdoms thread about grafting if you haven't already (I feel he does a better job of explaining all of the fine details)
Pretty much you just cut off an areole by cutting right behind it at an angle then cutting straight under it until you hit the other cut. The link up there has a picture that explains it better. Once cut the areole you can see a ring of vascular tissue that connects the areole to the main stem. After cutting the stock you just cut a seedling from its roots and stick it on there overlapping the vascular ring. The slime from the pereskiopsis will hold it in place. After that you just treat it like you would any other graft, throw it in the dark for a couple days and then after taking it out put it in a humidity chamber for a week or so. After that reintroduce it to normal conditions. With a little bit of luck they'll stick. Like anything else practice makes perfect the more you do it the easier it seems to get.
Hope someone finds this helpful.