The Mannequins modules work really nicely as a little system. If you were to go with a “complex voice” (2x Mangrove, Sisters, Just Friends, Cold Mac), my recommendation would be to grab another more standard VCA or mixer, so you can use Cold Mac for more interesting stuff. Something like Mutable Blinds would be good because you can use it as an offset/attenuverter as well. Obviously, this system would be more for subtractive (and some analogue FM) styles, so if you’re looking for digital/sampling/granular/etc, something else would be better for you.
Personally, I do think that, at least when you’re starting out, sometimes picking just one or two manufacturers can make it a little easier to get your head around the concepts of the modules, how they can work together and what they’re capable of, which is good for brands that people on Muffwiggler find “confusing” or “annoying” (read: interesting), like Mannequins or Make Noise.
I don’t have any experience integrating monome and modular stuff, but Ansible is top of my list when I do and would be a good choice if it works with your older grid/arc. They’re excellent additions because sequencers are often the largest modules, so monome modules can save you a lot of rack space.