Easiest to do is put the cameras all on their own PoE switch then connect the switch to their network, less mess than having 4-6 PoE injectors laying around on top of something with wires coming and going. Most 8 port PoE switches only have 4 PoE, the others are normal, there are PoE only switches out there or ones with higher PoE ratio's.
TRENDnet TPE-1020WS has a 10 port, 8 are PoE which allows for some growth. I use cisco SG100D-08P PoE switches to power my aeronet access points for my portable wireless setup for outside events.
I use 6 Cisco SGE2010P 48 port switches through out the building for our now 200+ camera system.