What you want to do is delete only OPT_FS.cfg (well I guess OPT_MFT.cfg, OPT_RF.cfg too as they're useless to you) and not the whole mm_config\ folder. This folder holds all of OPT's compatibility files, including the custom category. Also change the original OPT_Resource_Defaults.cfg so everywhere that says :NEEDS[!Firespitter,!RealFuels,!ModularFuelTanks] now says :NEEDS[!RealFuels,!ModularFuelTanks] this will cause that config not to disable the default tanks while Firespitter is install