I said it elsewhere but I personally have thought that their should have been a third category for forms named Environment that contained the environmental adaptions and mounts and affected your starting location. So you are well suited for a particular environment because that is where your form lives or you are well suited to using particular mounts because such creatures are common in your environment. So if you picked Unicorn mounts you would have a start that has stacks with Unicorns in your vicinity which would explain why your people are good at domesticating such creatures. Or if you picked desert preference your start would be in a desert like environment but there is no guarantee that there would be unicorns around. Basically your starting environment.
I am on the fence with this suggestion.
I think it introduces another problem. If we take mounts out of body traits and environmental adaptations out of mind traits and consolidate them in a third category - I'm with you that far - but in this case we could never have - for example - spider mounts and an underground start, or unicorns and an arctic start.
I think that this option also has more balance problems. For example - if we have the Body trait that offers +10hp to all our units and we stack it with a mount that offers +10hp - then we have +20hp on all mounted units. That is a very large bonus. How do we balance that? We might make them mutually exclusive, but that calls into question whether having +2 damage resistance is equivalent to +10hp and therefore should also be excluded if you choose a mount.
Then if we think we should remove the +10hp from the special mounts - do they then lose value compared to an environmental adaptation? Yes.
I'm just not sure that it offers a better balance - but I do think that both special mounts and environmental adaptations simply do not fit with the idea of 'body' and 'mind' traits. I don't want them removed from our list of options, but they are screaming for other categories by definition.
I like what you're saying, I do, but I've some questions in my mind about the best solution for these traits. They aren't like the others in their lists and I do want to be able to pick both at the same time. I do like the idea that it costs you something if you want the economic advantage of an economic and strategic advantage in a particular biome. I do like the idea that it costs something to have specialist mounts.
I'm not convinced a third category really helps us except that we get another way of making our form selection more nuanced and interesting - and on that point alone I want a third category - but I'm just not sure mounts and environmental adaptations are a balanced set of categories to lump together anymore than body traits and mounts, or mind traits and environmental adaptations.