I generally reserve the 10A outlets for the smallest loads that require the mechanical contacts to turn on and off, like my RODI input valve or BRS 1.1mL/min doser.
It does seem ironic that the largest capacity outlets are reserved for the smallest loads, but such is the case with TRIACs.
That being said, you should be fine running either your lights or the 200W heater, I run a two 300W heaters on two TRIAC outlets no problem. I also run my 4 bulb 54W T5 fixture, which is pretty close to your 6 bulb 39W fixture.
I think the TRIACs are rated to 5A? So 5 x 120 = 600W, but really, lets put a 70% derating in there to account for what the equipment vendors don't tell us, and call it about 420W. I would be comfortable putting an "advertised" 420W load on the TRIAC switches, and higher if I could confirm that the draw from the wall was less than 600W, not just the end rating of the device.
