Remove the head shim and running lower nitro should not present any problem. Re-tuning will be necessary any time anything is changed (low speed and high speed)

As far as adding petrol two stroke oil, don't do that. Just because you can get a petrol based two stroke oil to mix with methanol/ nitro fuel in small amounts does not mean it was designed for use in glow engines. It will increase the amount of hard carbon buildup inside your engine.
Stick with a castor or synthetic product intended for methanol or nitro. Blendzall, Klotz Techniplate, and some Maxim products will be more suitable. Ask around at a high performance motorcycle shop.

The other alternative is to use the standard airplane fuel and just insure you do not run it lean.