There is no authority which defines mini gardening, so you can do it in any way you want. Some of it is done outside, some inside, some looks like shrunken reality, some is pure fantasy. Some have fairies, but a mini garden doesn't have to be a "fairy garden." Personally, I say that I have a combo pot if there are no objects in it. The objects convert it to a mini garden, for me, but that's just a personal thing.
Suitable plants can come from any climate, from the deepest shade to full sun, leafy, cactus, or succulent, and be of any basic structural type (herbaceous, vine, shrub, tree). Mini gardens can be seasonal, or relatively permanent and changing with the seasons just like a full-scale garden.
Whether or not a particular plant would survive in a particular location or mini garden atmosphere, and for how long, would be up to the individual garden & gardener.
People build containers, structures, use a wild assortment of plants in countless different scenes, and some even have working trains or other activity. Longevity is not a requirement for all mini gardens, all mini gardeners. In exactly the same way that a full scale garden requires periodic maintenance, so do my mini gardens. I believe that most mini gardeners enjoy "gardening" their mini gardens. If I become unable to do ground-gardening at some point, I should still be able to trim, guide, move & propagate mini garden plants, I feel confident the weeds won't get out of hand. ; )
