On Earth, land vertebrates generally have four limbs. Other creatures such as insects can have more limbs, but those creatures tend to be small. I'm thinking of creating an alien ecosystem with relatively large, vertebrate-like creatures (larger than insects and with some kind of endoskeleton). Is it realistic for those creatures to have more than four limbs (six is good enough) and if so, what conditions would make it more likely for such a thing to evolve? To be clear, I am not interested in four-limbed creatures evolving extra limbs (from what I've read, that would be quite difficult, perhaps impossible), but creatures that have six or more limbs from their common ancestor.
I have seen some information on this topic, such as:
What seems to be missing much of the time, however, is numerical calculations. I would like to have, as much as possible, hard data on aspects such as stability (especially under different gravity conditions), a nervous system's ability to control multiple limbs, and the energy cost of extra limbs. Another issue that seems to be dealt with even less often is the number of fins that is useful for a fish or fish-like creature. As land-dwelling vertebrates on Earth evolved from a four-finned creature, and some similar process may happen elsewhere, especially on Earth-like planets, data about the usefulness of more fins to a fish-like creature should be useful as well.