Paying cumulative upkeep is always optional. If it's not paid, the permanent with cumulative upkeep is sacrificed. Partial payments of the total cumulative upkeep cost can't be made. For example, if a permanent with "cumulative upkeep {1}" has three age counters on it when its cumulative upkeep ability triggers, it gets another age counter and then its controller chooses to either pay {4} or sacrifice the permanent.

The rampage bonus is calculated only once per combat, when the triggered ability resolves. Adding or removing blockers later in combat won't change the bonus.

In a multiplayer game, you can put the Survivor tokens onto the battlefield under the control of different players.