According to the data sheet it should work. Have a look:
FlexStack-Plus is backward compatible with 2960-S and FlexStack. Mixing 2960-X and 2960-S members in a stack is supported. When the stack membership is mixed with 2960-X and 2960-S models, FlexStack-Plus falls back to FlexStack capabilities of 40Gbps stack bandwidth and four members maximum per stack.
link:
http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-2960-x-series-switches/white_paper_c11-728327.html
HTH