The CF6-50C family in the DC-10 had to live with reduced starter pneumatic pressure due to APU derating, but still started fine with fuel on at 10% N2. See if you can obtain GE's Specific Operating Instructions to be sure. The -50E core is identical to the -50C.
I believe the 15% requirement in the 747 was carryover (commonality) from early JT9's, which genuinely required 15% or more. Several operators had mixed fleets (GE & PW) and Boeing wanted to maintain a common procedure.