SoftDMC is a 'Smart' motion controller built into the FPGA. EMC does not support SoftDMC (or other 'Smart' motion controllers AFAIK).
EMC is based on real time Linux and performs all of the low level motion control tasks via software running on the host PC, therefore a smart motion controller would simply duplicate operations that EMC already performs very well.
There are some tasks where added hardware can improve EMCs performance. This 'Dumb' hardware includes PWM generators, DACs, Encoder counters, hardware step generation etc.
There are suitable 'Dumb' FPGA 5I20 configurations for running servo motors with EMC. For stepper motors, hardware step generation configurations for the 5I20 will soon be supported by EMC. Currently there is hardware step generation support for the 7I43 (parallel port connected FPGA card)
As far as jerky motion goes, is this with EMC? If so maybe a EMC expert can chime in here and help you with debugging ( checking latencies etc)