Don't overthink this one - concentrate on the important bits and the rest will fall into place or is just not that critical. Voltage is important. I would aim at a nominal supply voltage around 10-15% lower than the max driver voltage. For example, with my 80V drivers I run them from a nominal 68V supply. However, if you are using SMPS supplies then you are a bit restricted by what's available so a 36V supply would be safe for your 50V drivers where a 48V supply would be bordering disaster.

After that, think about current. This whole "is it RMS or peak?" question is always coming up, and I'm not even sure which mine mean when I am setting max current. Again, rule of thumb is that if the motors don't stall then there's enough current; if they get too hot then there's too much current. "Too hot" for a stepper means that it could be just a touch above "too hot to hold" but not much more. Assuming adequate ventilation, anyway.

So, what PSU rating do you then need? With a linear supply there is much more leeway as they are very happy with the kind of short-term pulse load from steppers as long as they can reasonably cope with the average. Because of the way they work, SMPS supplies cannot tolerate even short pulse overloads so need to be sized accordingly. Fortunately, it generally doesn't cost that much to go up a current rating or two if you are buying from China so you can err on the safe, over-specified, side. If you are likely to have multiple motions at once (seems plausible with your system) then just add the driver max current settings you are using and use that as the PSU rating, rounded up to make sure there is some headroom.

For the MCBs - I would expect that 5A would be enough for your system. That's a fair bit, but you could go up to 10A if not (and they aren't that expensive). Frankly, there isn't that great a choice of current ratings so it's not as if you can fine-tune anything in this area.

Keep the questions coming - I think that all the regulars on this site would rather answer a question than see someone get it wrong! And don't just listen to me - my views are not always completely in line with other people. I was trained as an electronic engineer but one thing you learn is that theory might suggest a good way to go, but you have to take practical aspects into account as well.