In general, I wouldn't recommend co-habitating vastly different species that occupy the same space within the enclosure. For the reasons stated above, and because they don't speak each other's language.
The rabbit tells the quail "Hey, that's my spot, get lost!" and the quail doesn't get it, so the rabbit might attack and the quail will probably move before it gets seriously hurt, but the quail is stressed because it feels it was attacked for no reason and the rabbit gets stressed because those stupid little feathered nuisances keep invading its space.
It could be the quail that are pecking the rabbit or they might not show any aggression at all, but the point is, they don't understand each other and shouldn't be forced to stay near each other in limited space.
It's a different matter if we are talking a tree climbing species with a ground dwelling species, for instance. You don't limit the available space for either by adding another species and you don't force them to be near someone they don't understand. You give them the option, and in some cases they might not be compatible - if the tree climbing species attacks the ground dweller they'll need to be separated. If one prefers the food of the other and this isn't appropriate and can't be made inaccessible. But if they are compatible, you've managed to save space without reducing the welfare of the animals.
If you want to do it with two ground dwelling species, I'd give them significantly more space than I would have if I were to keep each species separately. And that's not 2 sqft/quail and 10 sqft/rabbit - that's maybe 10 sqft/quail and 50 sqft/rabbit. And then you loose the advantage of saving space, you still risk unpleasant encounters among the animals and the only advantages you are left with are the chance that the animals might gain something from the presence of each other.. Which they might, but I strongly doubt the presence of a tortoise would enrich the quail for more than a week. Once they are used to it, you might as well have a rock in their enclosure that you move every day. Which you might well have to resort to anyway, as it is my understanding that tortoises are pretty sensitive to dust, dirt and such, which you are likely to find in a quail enclosure..
I'm not saying it absolutely can't be done. And there might be species where it can actually be an advantage to both you and the animals. But I think they are few and far between.