You're really on a tear aren't you :-)! Terrific! Chopping means to take a tree of any size and cut it into two pieces (of any size), though it's usually done with a relatively young one of e.g. 5+ (and +) feet that looks bonsai worthy - i.e. has a nice balanced root spread, or else at least a decent sized lower trunk tapering to the higher parts, and some 'movement' in the lower trunk - not straight up and down (unless you like formal upright styles). You'd throw away the top half (unless you're smart enough to have airlayered it off properly) and work with what's left. The only caution would be to know the species as some won't bud back lower than the already existing lowest branch(es). Coleus is a bit of an experiment to say the least, but it's definitely been done, so go ahead. Jade are done more often, but again are not traditional, and you wouldn't want to wire them for fear of cutting into the trunk, or snapping branches... graduated weights work better if at all. You'd also not clip leaves, but use a tight twist right at the trunk to remove them. If you can make coleus work on a rock, try it! Marigolds do seem to be a stretch however - do they ever get woody? But use them as accent plants nearby, or just a few sprigs in a bonsai pot could look interesting after all.