For a stack of thyristors, is the "critical rate of rise of off-state voltage" scalable with the number of thyristors?
Note that I'm not an expert in thyristors specifically, which is why this answer sounds general.

The general rule for stacking things like this is that it's your responsibility to make sure that the voltage is balanced (or current, if it's a bunch of devices in parallel).  This means that you either need to look for the manufacturer's recommendations, that you need to read their data sheets with a careful and critical eye and do some analysis, and possibly that you need to add components to balance the string (like a cap, resistor, or both in parallel with each device to help them stay balanced, at the cost of worse parasitics in the stack).