so a ghoul outside of underworld mentions that Super Mutants don't mess with the ghouls, possibly because they're sort of "kin", but do you think its more likely that they can't turn ghouls into super mutants?...just a thought

That is possible. On both counts. They are both mutants, just with different methods of creation. And the process of ghoulification might render one immune to FEV. Or perhaps just insure that you die if exposed to it rather than mutate further?

However, Super Mutants will attack Brahmins, who are also mutants. So mutant solidarity only seems to go so far. I am not sure about other mutated animals though.

so a ghoul outside of underworld mentions that Super Mutants don't mess with the ghouls

but do you think its more likely that they can't turn ghouls into super mutants?...just a thought

That is possible. On both counts. They are both mutants, just with different methods of creation. And the process of ghoulification might render one immune to FEV. Or perhaps just insure that you die if exposed to it rather than mutate further?

so a ghoul outside of underworld mentions that Super Mutants don't mess with the ghouls, possibly because they're sort of "kin",

That's only partly true. Super Mutants will still attack Feral Ghouls, just not the ones that still have their wits about them.

If you talk to Tulip in Underworld, she will tell you that whatever makes them what they are, doesn't work on them. This leads me to believe that it may have been tried before and didn't work, Don't know anything about the dying part though.

However, creating new Super Mutants proved to be a hit-or-miss process. The great majority of Super Mutants produced by the Master, and later his lieutenant, in the Mariposa base's FEV vats were big, dumb brutes. Physically, they were vastly superior to pure-strain humans, but they possessed the intelligence of children. The Master was certain this mutational randomness was related to the amount of radiation damage suffered by an individual: humans who had not been exposed to much of the post-nuclear environment's radiation produced more intelligent Mutants after their FEV infection (see his personal diary for his thoughts on this). His lieutenant, however, had a different theory. When the Chinese nuclear bombs hit the original West Tek Research Facility and turned it into the region known only as the Glow, the impact of the nuclear shockwave cracked open a few nutrient solution tanks filled with FEV. The nuclear bombs' radiation then mutated this FEV into an airborne strain of the original virus, but this new airborne strain of the FEV did not have any real mutagenic effects on human beings. All it did was inoculate any person infected with it against the original, mutational strain of the FEV, acting as a form of vaccine and giving exposed creatures some limited resistance to the FEV's mutational effects

Yeah but they're just doing that cuz the ferrals are crazed and would attack them anyways- they're not looking for ferrals to fight just fighting them when they have to