I have some prototyping boards like this:-
It's a standard 0.1 inch pitch and I like them. Is this suitable for a medium voltage of about 200 V DC? I'm concerned that the voltage might just repetitively creep /jump between pads right across the board.
No, don't put 200 V between adjacent pads.
When nothing is going wrong, it should work. It varies with pressure, temperature, and humidity, but air can roughly hold off 1 kV per mm. Using that as a guide, 200 V would be 200 µm or 8 mils. Most likely you have 10-20 mils between pads.
However, since failure can be dangerous and cause all kinds of undesirable things, you want to be conservative. First, the 8 mil rough guide is the expect-to-fail point. Obviously you don't want to be there. It doesn't take much dirt or accidental solder ball or something to turn a 20 mil gap into a 8 mil gap or less.
Usually you want ¼ inch or 5 mm between line voltage and anything a human can touch. That's deliberately very conservative, but again, that's because the cost of failure is so high.
Don't use a breadboard like that for anything you're not willing to touch directly, which pretty much means stay below 48 V.
TLDR: treat higher DC voltages with great respect.
DC power at high voltage is a more ferocious creature than low voltage DC or even high voltage AC. AC reverses polarity every 9ms, dropping to 0 volts and 0 amps, which creates an opportunity to snuff an arc. That is also true for directly rectified DC - but not at all, once it is smoothed out to continuous DC.
It's a good thing to think about in design.
8 mils may hold off an arc from starting under ideal conditions, but once that arc does strike, it'll take a lot more gap to extinguish it. It will indeed vaporize conductor material until it has widened enough gap. Here's a video of 600VDC doing that to a streetcar.
DC contactors have blowouts designed to snuff the arc (or more accurately, throw the plasma path into an arc chute which cuts it up). They use permanent magnets, residual magnetism from the collapse of the coil field, or a blast of air. This page shows a photo of a contactor rated to 300VDC, only 50% more than your design, and you see the not inconsiderable arc suppression gear.
Of course, semiconductors take a different approach to circuit interruption, but if your semiconductor is interrupting anything inductive, you may have a voltage spike. That could find an insulation weakness and strike an arc which a lower voltage could maintain.
To prevent these arcs from starting, you want insulation that might be appropriate to a much higher AC voltage.
You might also consider arc-fault detection on the DC power supply.
