Add that other breaker it won't hurt when you add things later. payn4school, Yes you can put those 2 15 amps together, IF and only if they are on the same phase in your pannel. If they are dif phase Kaboom. If you have a 100 or 200 amp pannel then you have 2 phases that's how you get 220. I would just run off the 15 amps, depending on how many lights you have.

Ok, here's what I run on 20 amps and pushing it. I was residential electric for 20 years. A 1000 watt hortilux eye, A 6000 btu Ac, A dehumidifier for a whole house. Vent fans in and out. A phototron and 4 , 4 foot fluro's, and 4 small pumps for hydro units and a fan to blow on plants. I'm pushing it now , so I'm running a 30 amp 220 or 221 whatever it takes. LOL.

One thing do not oversize the breaker on the wire. That is what burns you up. A breaker or fuse will blow before the wire burns for safety.