the biggest issue when merging mods is the playerRACE.entity's and the galaxyscenariodef.
as both the sgi and sinsplus BOTH have customised playerRACE.entitiy's and the galaxyscenariodef, the best advice I can give is to add the sinsplus extras to the sgi copies, then re-build the manifests for the combined mod, then test with the dev.exe in the game folder, and for most of the work you will eant to use notpad++
and one helpful idea for testing is make ONE change then test,yes it is slow but with only one change you at least have a chance to find and fix the problems
harpo