Named characters cannot take artefacts or command traits.
You'll want to make a different model your general usually, otherwise you essentially lose the command trait. Though some named characters have special abilities that only apply if they are your General, so it's not always a given.
I'm pretty sure that mount traits are Orruk warclan lists only, and Troggbosses making Troggoths Battleline is Gloomspite lists only. Neither of these abilities are on the warscroll, only found in the respective books.
You generally lose out on a lot of useful abilities if you take a Grand Allegiance. Its a totally valid choice for more casual or narrative games, but if you are playing in a more competitive environment it's not a good idea and you'll be better off building an army based on a specific Battletome.