Gotten my core to +13 and trying to find some way to boost it further
Is it possible to use +1 gear boosters like you do -1 gear boosters with EB2 and pray for great success?
Or are they completely useless on +13> gear?
+1 boosters have no effect on +13 items and above.
I believe eb2 is a separate effect that takes priority over whatever fuel you use. So as long as you can use with that fuel type, you should have chance of +2 provided it will not put your gear over the cap (currently +15 cap)
Above is correct.
If you had a gear at +13 and you used +1 gear booster with an EB2, you have a chance at +15.
Friend got a 14 with a +1 booster and an EB2 from +12, for reference.