I am wondering if it is okay to untucked boxes that I don't you. For instance if I'm flying PNW can I untick my FSX global and airports I'm not currently using? I am trying to avoid OOM which so far I keep getting. I know FEX has to load all those sceneries even if I'm not flying them which uses extra VAS.

I believe your assumption is not correct.  True, the sceneries are active, but the sim doesn't load them until it needs them.  Only until the airport in question is within the LOD radius you have set in the config file will the airport be loaded.  So for instance, if you're flying in PNW and never fly anywhere near KSFO, there is no effect on VAS, since KSFO is never loaded.  FTX Global (I assume you meant FTX, not FSX) doesn't enter into the problem either.  It's a non issue in any case.