I need some advice here. You may also know some of the aircraft have pretty bad performance if fuel load is 100%. Examples are P51 and F4u. However the default fuel load is 100% for all aircraft in game. I know you can change the fuel load for your unit, but not for other units. Now my sister units equipped with 100%loaded p51s are getting shot out of sky like no tomorrow. Is there any way to change the default load by aircraft?

Yep, via timetable.dcg, again:

 

19420101 SetFuel II_JG10 = 75

 

You have to set it only once for each squad, preferably at campaign start (if you want to start a new campaign). Now you know where and how to put it into the file

 

Syntax:

 

[Date from which the command will be effective]_SetFuel_[squadId]_=_[Fuel percentage]

 

 

So once you set it once at the beginning of the campaign, it stays the same for the entire campaign right? I don't have set and reset every mission right?

Exactly, you change it once and for all, and it remains the same until you change it again. For a long range fighter like the P51 50% is absolutely safe, but 25% will be OK as well.