What mixer(s) specifically can be used with an MSR-16 in this manner?
Can summing boxes like ones from Dangerous Music be used with this machine to monitor?
The beauty of using a small mixer for front-end and a bigger one for mixdown is that anything with enough channels will do the job. Otherwise, you'd have to find a proper recording mixer with switchable inputs which will be rare and expensive since most people are using DAWs these days. The other approach is to get a live mixer with far more channels than you would otherwise need so that you can use 16 for mixdown and 8 for front-end.
As I was saying at the time, a lot depends on how many inputs you need to record at once. If you're only ever going to be recording one instrument at a time, you can use a 4-way Y lead. If you are going to be recording multiple instruments live, you'll need a front-end mixer with lots of output groups. In either case, a patchbay is likely to help immensely.
At the time I was originally replying to the thread, I was probably using a Yamaha MG14 as front-end mixer, and a 20-channel Soundcraft MPXi20. These days I'm using a stack of Behringer RX1602 1U mixers as the front-end, because it saves space. The drawback is that it has no EQ or preamp, and although it has FX send, the return will eat up one of the channels. I have three which I'm chaining.
For mixdown I'm using an Allen & Heath GL2400-24.
I've never heard of the Dangerous Music unit before, but given that it costs about as much as I paid for a professional 24-track deck, I suspect it would be colossal overkill.