SCRs do not parallel well. Semiconductor junctions, like those in SCRs, diodes, and bipolar transistors, have a forward voltage drop that decreases with temperature. The hotter SCR will therefore draw more of the current making it even hotter, drawing more of the current, etc.
Why do you think you need SCRs? Their main attributes is their latching behavior and the fact that they can be produced with large current capabilities. If you just want the latter, several MOSFETs in parallel would work. These have a positive temperature coefficient so don't exhibit thermal runaway. Still, you need to derate from assuming each of the FETs will share the current equally.
Since FETs look mostly resistive when on, parallelling them not only decreases the dissipation on each, but also the total dissipation. At 200 A, only 5 mΩ will cause a 1 V drop and 200 W dissipation. That won't be trivial to design to no matter what you use as the switch. It will help if this 200 A is only in short pulses with a much lower averge. Take a look at the SCR datasheet and see the forward drop at 200 A. It won't be all that nice either, and you'll have to deal with significant dissipation with the SCR too.
Fortunately 5 mΩ is not that far fetched for a few FETs in parallel. 4 FETs with maximum Rdson of 20 mΩ would do it, and each would dissipate only 50 W if they were to share the current equally. I'd probably derate to 100 W per FET when designing the thermal system.
A reason SCRs aren't used for push-pull schemes involving high frequencies, as far as I am aware is the abominable turn-off time of the things.
Gate trigger to conductance can be in the micro-second range for most types, but for many types that I am aware of turn off times are in the 10 to 100us range, which is pretty much the death of any efficient use of a square wave or PWM signal for power conversion.
There may be whole series of SCRs that are super fast in both regards, but I haven't heard of them.
EDIT: (In SCR's turn-off time is not the transition of stopping to conduct, which would not make much sense anyway, but the time the device is expected to be "relaxed" before a voltage is again applied that might induce current over its holding current. This is the time needed for the blocking diode barrier to regain its withstand voltage. So some tweaking may be possible by using an SCR rated for 200V at 40V, but it won't be an order of magnitude.)
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In a former life where I worked in power electronics we would not gate the SCRs on in reverse conduction because it reduced the reverse blocking capability of the SCR. It would therefore be easier for an inductive spike or other transient to force the SCR on and ultimately lead to its destruction.
I don't have any evidence to back this up, it was "tribal knowledge" from our greybeard engineers. We were also working on medium voltage (up to 4kV) and high voltage (13kV and up) systems where we had a number of SCRs in series and it was very important to maintain reverse blocking voltage.