The second part of your question, about Slam damage, has a canonical answer.
The Gamemaster may limit your base slam attack damage (before applying circumstance modifiers) by the series power level.
Example: Your hero flies into a foe, moving at speed rank 10. His unarmed damage (Strength) rank is only 2, so he uses his speed rank of 10 for the damage. Since he also moved his full speed to build up momentum, he increases his damage by +1 for a total damage rank of 11. If a base damage rank of 10 is too high for the series, the GM may impose a lower limit on his slam attack damage, applying the +1 modifier for the full speed move to the lowered rank.
So, in short, there's nothing guaranteeing you a chance to break PL, especially if you have a character built to avoid any of the damage from doing the attack.
As regards reaction "attacks" by linking it to movement, the onus is on the GM to police abusive builds like that.
The GM is the final arbiter as to whether or not a triggering circumstance is suitable for a particular effect in the context of the series. Caution should be used to keep this option from being abused.