Can i get away with a pcb heatsink for a triac?
According to Fig. 5 &amp; Fig. 11 in the datasheet the power dissipation will be about 0.95 W. 

You will need about 2 sq inch.  This means top and bottom.  There should be thermal vias where the device thermal pad is solder to the PCB pad. 

I would use the TO-263AB because it will be easier to solder the thermal pad to the PCB.  

If you use the TO-220 you still need a thermal pad on the PCB to solder it to. Screw it down before soldering.  Use solder paste if you have some. 





In the app note below, section 3.1.1 Example: Calculating the Required Board Size to Hit a Target θJA is very similar to your project.

This example is for a 0.94 W device with a θJC of 7.3°C W. It required 2.23 sq. in. for a target θJA of 40°c.  Using the same calculations with a θJC of 1.9°C/W the area was reduced to 1.91 sq. in. 

So 2 sq. in. sounds like a safe bet with about 25°C margin.   

APP NOTE: Thermal Design By Insight, Not Hindsight