Is there a way to know if the photocell is a switch or a dimmer?
There are certain photocells that are only rated for incandescent bulbs and there are also photocells that are rated for L.E.D and flourescent bulbs. You would have to make sure if yours will allow L.E.D.s The same goes with dimmer switches. A dimmer that is only rated with incandescent bulbs, if hooked up to a led or flourescent (cfl) bulb will eventually burn out. Again you have to make sure u find the correct photo cell. also to you will install the photocell between the switch and the 1st light in the series of light. It will be 

Power comming into the switch > power from the switch to the photo cell > power from the photo cell to the light and so on....