How can SCRs be effectively paralleled?
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 m&Omega; 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 m&Omega; is not that far fetched for a few FETs in parallel.  4 FETs with maximum Rdson of 20 m&Omega; 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.