I'm not sure about bringing a solo Doombull; I feel like he's not that killy or durable on his own for his 120 points. He's when accompanied by other Warherd he's great, but I think it would be a waste of a CP to boost a solo Doombull. I'd probably be better served by taking another 10 bestigors for the same points. The Bestigors bring 10 wounds vs. 8, 4+ vs. 5+ armour save, 20 attacks vs. 3--5 attacks (though he can get up to 10 with Bloodgreed if I get really lucky).
I feel like the Jabberslythe as an ally brings something new to the table compared to the Doombull or more Bestigors, like mortal wounds, ranged attack, nerfing aura etc.
List building rules are just the standard for 1250 points (so 2 battleline), and the tournament organizers just announced today that all of the Malign Sorcery stuff is in (realms of battle, etc.)