If an effect refers to a Food, it means any Food artifact, not just a Food artifact token. For example, you can sacrifice Tough Cookie (an Artifact Creature — Food Golem) to activate Maraleaf Rider's ability (an ability with "Sacrifice a Food" in its cost).

Food is an artifact type. Even though it appears on some creatures, it's never a creature type.

You can't sacrifice a Food to pay multiple costs. For example, you can't sacrifice a Food token to activate its own ability and also to activate Maraleaf Rider's ability.

Whatever you do, don't eat the delicious cards.

Bill the Pony's last ability doesn't actually change any creature's power. It changes only the amount of combat damage it assigns. All other rules and effects that check power or toughness use the real values. For example, since having two creatures fight doesn't result in combat damage, an effect that causes the affected creature to fight another creature will still use its power to determine how much damage is dealt.