I have great sucess grafting plants that size roughly 1.5cm on to Trichocereus cacti , and smaller seedlings (as well as older ) onto Pereskiopsis spathulata which is great for Lophophora's!
For Trichocereus just make sure the Vascular Rings are crossed at 2 points, and trim back the root stock so it doest lift the button off! a section of womens panty hose works something nice to secure the button to the root stock and easily hooks on the spines of the rootstock!
Depends on how much of the growth portion is left on the young roots,if your able to leave "aerial nodes" -- areoles it will push pups out of those.
Alternatively if you can leave the areoles on the bottom you could make two grafts with one plant, and graft the lower portion as well.. food for thought
While grafted plants are super interesting, and grow extreamly fast, Lophophora on there own roots are my main goal. Not sure how many you have but i would experiment with what you would concider acceptable loss. If you don't gamble you can't win!