They look a little like flood versions of trees. With fleshy sacks on them. There are some off to the right of your starting base.
yes
Yes.
No. The Flood is controlled by an AI (Artificial Intellgence). However, you can play as the Covenant in Skirmish.
No, the only teams you can be in Halo Wars are the Marines (humans) and the Covenant.
NO, but maybe there is a cheat for it