Conventional Hydronic Boiler with multi-zones

I am retrofitting multiple zones into my current system and I was thinking that the boiler could be shocked with cold water from 1 zone with hot water on already and another zone of cold water being turned on. I will have a zone thats always on so I may not even need a mixing valve... not sure. So my questions are as follows

1. Do I even need a mixing valve with one zone always on?
2. Should I use a 3 way mixing valve, 4 way mixing valve, or hydronic separator?
3. I read something about close Ts, is that even related to this?
4. Can I get away with only using one pump?
5.