It's not possible to source transform a lone ideal source. Other than that, you can use it for pretty much anything. However, I will also note that the layout of the circuit will definitely determine what makes the most sense.
What you have to look at is what you get after performing the transformation, and whether or not that will help simplify the circuit. Thevenin equivalents are series while Norton equivalents are parallel, so you have to look at where changing series elements into parallel or vise-versa will help. Generally, if you have two blocks in series, try to transform them to thevenin equivalents so you can combine the sources and resistances. If you have two blocks in parallel, use the Norton equivalent so the sources and resistances can be combined in parallel.
In this case, transforming the 2A source and 4 ohm resistor, then combining with the 12V source and 6 ohm resistor, then transforming one more time to get a norton equivalent is probably the most straightforward sequence. You can source transform the 6 ohm and 12 volt sources first, but that doesn't really make combining elements much easier.