The minor allele should be determined based on all populations combined for SFS constructed from more than 2 populations:
https://groups.google.com/forum/#!searchin/fastsimcoal/sfs$20multiple$20populations%7Csort:date/fastsimcoal/zWO_ERhHjOg/cnPXsDCXjRAJ
EDIT: This is only a problem for unfolded SFS, folded SFS doesn't care about minor allele.