Meh, I'm probably just going to go the traditional fantasy route and make them beastmen with hands. But it's an interesting thought experiment. Many primates live in large groups. I wouldn't say they evolved naturally. Most or all races in my world are deliberately or accidentally designed, not always for the roles they actually have.
In that case, there is no issue at all. Since the species were all designed to be a particular way, you can make them however you want. As a reader I would be wondering why this had happened - why give tool use, a skill which should prove advantageous, to one species, but then engineer a situation in which they can only use that skill for the benefit of another species? I’m not saying there is anything wrong with that scenario, I would just want to know what the motive was for something creating that situation in the first place. Even if it’s just as a fucked up experiment to see what would happen.
I doubt that the parties responsible actually made the beasts so that they could build a civilization based on human slavery. It's a kind of Lovecraftian world, with alchemists, demons and higher beings creating life for arbitrary purposes.
Hm, well I don’t read the genre so don’t really understand how that mechanic would work. But it sounds like you’ve found a way of getting the species in question into that situation while circumventing the problems associated with them evolving that way.
The most likely origin for such a system, in my opinion, is that the animals and humans developed some sort of symbiotic relationship earlier in their evolutionary histories. The humans might have performed thumb-requisite tasks for the animals, while the animals performed some other function, possibly providing protection. The animals might have evolved an intellect equivalent to that of the humans in order to be able to communicate with their symbiotes and maintain a relatively even relationship. Over time, if the animals evolved to be stronger than and at least as smart as the humans, they might have managed to gain superiority through strength--a "do what we say or we'll kill you" sort of situation to replace the original symbiosis. This would answer the concerns of other posters on this thread asking why the animals would want tools if they'd never used them before enslaving humans: they've always relied on thumb-wielding humans to use tools, write down information, etc. and thus evolved the need for tools without the need to use tools themselves. The humans always helped out with things that required thumbs; it's just that now the animals have gained the upper hand and shifted from partners to masters--or, if we're using biological terms since this is a very evolution-based topic, symbiotes to parasites.
If your animal people are decently intelligent, I don't see why they wouldn't be able to build things or use technology. With enough creativity, there's no reason why they can't overcome any disadvantage their anatomy presents. Thumbs are great, but seriously over rated. Try using any tool or operate any appliance in your home without your thumbs and its really not that difficult. The only thing I had serious trouble with was a hammer and an X-Box. Don't believe me? Check out J.C. Ryan, a Master Penman and calligrapher who had no hands.
