will a 120mm fan fit in a 120/140mm without a bracket

Here are the fan mounts, copied from CM's site for that case.

Front: 140mm fan x 1 (optional)
or 120mm fan x 2 (optional)
Rear: 120mm black fan x 1 (pre-installed)
Side (left): 120mm or 140mm fan x 1 (optional)
Side (right): 120mm fan x 2 (optional)