Other thoughts:
You almost certainly have this planned already, but it definitely seems like more ought to be done with souls, particularly with drawing a distinction between sentient and animal souls, the way that orc forts do. I would go so far as to say that the unit upgrades, at least the more interesting ones, should probably require sentient souls, rather than being able to fully upgrade your foocubi using only the butchery of dumb animals.
Beyond that. The development path for foocubi forts is fairly short, at present. Dwarves can research new building types, Orcs can dreamwalk or raid for them, Kobolds can steal plans, gnomes can...I don't know, gnomes haven't really clicked for me, and warlocks don't particularly appeal. But foocubi don't have any mechanics like that at present, and they definitely ought to, ideally.
Perhaps...hm. I'm kind of thinking as I write here, but another minor issue for me is that I don't usually even feel inclined to make extensive use of the conversion mechanics, because I don't want to subject my fort to massive population growth. I prefer to keep things kind of small, capping migrants around the 40-60 range, and so capturing everyone to make them join my ranks doesn't greatly appeal. Perhaps if you had a choice between using a captured prisoner for that purpos, or using them instead to become a special resource that could be used when "bargaining with Hell," or something like that? (I assume this kind of creature -> item conversion is possible mechanically). You could use souls of the fallen to empower your demons, but to access special buildings and advanced weapons and armor (and MATERIALS, another major issue), you'd have to capture invaders alive.
On a similar note, given the theoretical emphasis on capturing people, it would be nice if foocubi had access to a weapon that does it, like gnomes and goblins(?) do. And I guess humans have access as well, by buying them. Tossing a net?
As far as foocubus weapons go, the whole whips and swords thing is fine, very good. But then their "best" weapon seems to be the stygian pitchfork, which just...blah, I don't know. A pitchfork that shoots magic shards? It doesn't match the rest of the aesthetic, and it feels more than slightly bizarre. My first instinct is that, especially with the rest of the design I've seen, the bulk of their ranged abilities should be from their inherent magic. They toss a fireball on the approach, or entrance their target with an enigmatic smirk, not shoot a pitchfork at them. If they have to use a ranged weapon, ordinary bows are fine - there's some overlap there with the image of an Amazon. Upgrading basic weaponry by infusing it with sentient souls is an excellent notion, but the result of that should be something that fits the theme. More interesting names combined with moderately improved attacks (and maybe a small chance of radical damage, like some special dwarven weapons have) would probably be more than enough. Demon whip. Living blade. Burning spear. Something like that.
Other possible thematically appropriate foocubus weapons: As above, the spear seems a decent one to focus on, congruent with the Amazon comparison. Also, maybe fingernail blades?
As for armor: Obviously, succubi should not be wearing much. You've got the corset, which works, but everything else is pretty normal, so these gals are putting on full helms and heavy boots and such. Greaves are actually pretty appropriate, in terms of what the name historically refers to, but DF has them in kind of a "heavy armor" style niche, it feels like. Or maybe I'm misinterpreting that. In any case, I'd say that foocubus armor should be relatively low coverage, named as being very light ("crown," perhaps, and "wrist guards," and maybe "armored skirt" if we don't like the associations of greaves.)
Hipposandal could be a good boot replacement for a hoofed people, if it weren't for the unfortunate name. Low layer size, but obviously all of them should be SHAPED, both for thematic reasons and to keep from massive, ridiculous stacking. Upgraded versions (another use for souls) could maintain the low size, but increase coverage to full, because MAGIC.
Materials, hooboy. Obviously, foocubi have little real choice for their metals. Stygian bronze, despite having an awesomely thematic name, is an early-tier material at best, being basically only as good as iron. Basiliskine is the only long-term option. (Disregarding the always-popular "melt down elven mithril and use that). And it's a more than decent option, quality-wise. But presumably we want foocubi to lean towards wearing something a little more demony.
Proposed solution: Make stygian bronze a
heavy, (since when is hell associated with LIGHT things?) extremely sturdy metal that requires souls to create, as well as flux or whatever. Basic souls should work here. Combining with the low layer size of the armor, you could have a relatively normal amount of total weight for a fully-armored soldier, and mixed protection - powerful but prone to failure with the low-coverage versions, or just powerful overall once everything is upgraded with souls.
Leather, of course, is a very thematic material for them to work with, and especially to make the bodysuits out of. But foocubi don't have any special ones. I don't know if it should be a new reaction in the tanner, or in the soul forge, or somewhere else entirely, but I would suggest perhaps "Bloodleather," made from leather, souls, and blood, as a light material roughly equivalent to steel in protection.
Altogether, this would drastically increase the demand for souls. Perhaps you could also 'downgrade,' exchange a sentient soul for multiple basic souls to use in such material reactions. And, ah. I guess that's all I've got, right now.