When you say "enemy towards everyone" do you mean "enemy towards BLUFOR and INDEPENDENT"?  Because that's simple.  If you want them enemy against OPFOR as well, then you're designing a poor mission. :)   To make only Syndikat units hostile to B and I you need to place down a higher ranked OPFOR unit first, then group the Syndikat to them.  Set the OPFOR unit to 0% Probability of Presence by expanding Object: Presence in it's attributes and setting the bar far left.  That unit won't actually show up in game but will make the group they were leader of count as OPFOR.  You'll see them outlined in red in the editor if you did it correctly.   Actually, looks like in Eden you can delete the OPFOR unit and they remain OPFOR.  Hmm.  Probably safest to leave them there as 0% though.