Maybe if I talk about my personal experience with 7 player games this could help.
I used
Krenko, Mob Boss in some of the games. The issue is that I had to have an early win. Mono-red really can't play the long game, especially not in an 8 person pod. Too many things change by the time your turn comes around.
Instead I used
Captain Sisay. Her early game wins we're way more consistent, and she could last into the late game if needed. The issue is that it wasn't needed; she was too powerful, and most people in that group were using low interaction decks. I stopped using her out of fairness.
Kynaios and Tiro of Meletis gave the best results. I dubbed the deck "specific-hug" because it emphasized on helping my partner and myself only (using cards like
Intellectual Offering,
Skullwinder, and
Guided Passage). The combo was pretty subtle, which I think is what gave this deck the most mileage. The deck was entirely ramp and card draw until the combo turn. It was
Kiki-Jiki, Mirror Breaker and
Felidar Guardian, but the deck had multiple ways of building into it (
Tooth and Nail,
Birthing Pod chain). Maybe something like this would work for you.
Actually, if you give me an idea of the meta, I might be able to help more. Specifically, how often do you see combo decks, stax decks, and aggro decks (approximate number of players)? Are games usually won with a combo, or incremental damage over time? How much interaction is there? Are people usually able to stop a combo when it's set in motion? And which of the decks you see are the most threatening?