You should keep the Neutron in its own case, giving you room for support modules which you will need.
How have you patched the Dot and the Black Sequencer? I don't have either, but I don't see a way of doing what you want to do with just these two modules. The BS expects a regular clock and has only one Gate In. With a VCA, you could drop out notes from the BS on low gate from the other Dot channel. Other sequencers would let you use a Dot channel as an irregular clock, but mostly only one channel, not two. It might make more sense to use the Dot separately on percussive voices, and the BS on melodic ones.
If you want to combine two or more audio signals, then, yes, you will need a mixer (perhaps more than one, depending on what you want to do), and there are many possibilities. The µJack is discontinued, but Intellijel makes the Outs module, and again there are other possibilities. I have personal experience with the ALM HPO and Befaco Out v3, and like both.
There are more modules you are likely to need, but it might be best to start using these while you keep doing research. Through use you may discover things you want to do and can look for ways to accomplish them.