Check the PoE brick specifications to ensure it is up to the task.  Your limitation may be the PoE budget of 1A per port and 2A max.  I don't know what juice your AP requires or a PoE camera would need.

As long as you are happy with the 100mbps performance of the RB750, I'd give it a go.  Perhaps ask in the Offers and Wanted forum if anyone has a retired/spare RB750.

Learn the difference between passive PoE that Mikrotik supports and 802.3af/at.