My question is just if it is possible to change the licensing terms as a seller in fab or does it have to be one of the listed licenses and or does one of the listed licenses allow for adding own licensing terms?
No, It is not Possible to Use Custom License or Change Terms of License, Seller have to choose appropriate License for their Listings given by Epic Games.
So if there was a asset that was assigned to be standard licensing but in the description said to need a different license to use that product, i could freely use the product without needing the different license mentioned?
The Asset Published on Fab is only Governed by the Selection of Epic Licenses and Seller agrees to that in there Agreement, You don’t have to worry about anything else if Bought in Fab.
Although I am not sure how it Slips from there Review or Verification Process (Most probably it might be Sketchfab Asset not Unreal).
But do look out for “Ripped” or “Copyrighted” content, That may lead you in Trouble. Because Epic doesn’t guarantee whether Content is 100% Legit or not, on there Buyer/Customer EULA.