You can use the SC-1 with Legacy, it won't notice any difference, Legacy talks to switches in TMCC mode. As long as the SC-1 boxes are working for you, you can keep them.
The STM-2 allows LCS to see the actual position of the switches, even if the non-derailing has switched them. If you're going to use something like an iPad to control the layout switches and power districts, then the STM-2 would be useful.