100% agreed. Just to clarify: lobstercreed means to add the 10G port to the existing aggregation.
If you use the 10G port outside the LACP aggregation, you would need to rewrite all policies as well.
You'll notice the most convenient feature of an LACP trunk is that you can add or subtract member ports without any further disruption.
BTW, I seem to remember that STP should be disabled on FGT links anyway.
"Kernel panic: Aiee, killing interrupt handler!"