Euphorbia milli - are not often used for bonsai. They can be trained, to make attractive house plants. To some degree they can be trained to be used as an accent plant where the specimen is enjoyed for its flowers more than for being tree like. Kusamono or Sanyasou are the terms for the type of accent plants I am thinking of.
Many succulents are referred to as "bonsai-able" often by other succulent growers, not usually by the bonsai community. E. milli is not very tree like, it is very upright, but does not form many branches, and key to the tree image, branches having branches. You can not get the number of degrees of ramification that normally go with a tree image, versus a shrub image.
But I like them, and they look great when grown in a bonsai pot with colors to contrast the flower colors. I like green and blue glazes to complement the red to greenish white flowers depending on which cultivar you have.
Feel free to post pictures of yours, even if it is not '"traditional Bonsai" we do like seeing well grown plants. Post in the Flowering sub forum or the New to Bonsai sub forum. Or in the Kusamono sub-forum.