Because Char-Rumber's power is normally negative, you'll have to give it at least +2/+0 before it'll deal damage. It won't deal negative damage.

Yes, Char-Rumbler's printed power is -1. While its power is -1 or 0, it simply deals no combat damage.

A first strike damage step will be created if Char-Rumbler is in combat, even if its power is 0 or less. The normal combat damage step will follow.

If Char-Rumbler's power changes between the assignment of first-strike combat damage and the assignment of normal combat damage, Char-Rumbler will deal a different amount of damage in each combat damage step.