Are the Taz 6 and Taz 5 Tool Heads interchangeable? by that I mean the same carriage? I need to build another Tool head for a smaller nozzle and wondered if I should just build the Taz 6 Version With the 3 40mm Fans.
Sort of. The left hand fillament cooling fan will interfere with the Taz 5 Z height adjustment target and Z min limit switch. You can either replace it with one at a different angle (there should be some on Thingiverse I think) or add a spacer to limit X travel by 3-4mm. The Taz 6 toolhead also has a different pinout than the earlier Taz 5 one so you may need an adaptor, but Lulzbot sells those too. The Taz 5 also won’t have the X maximum endstop, but that won’t matter. The carriage will fit in all other respects. The Taz 6 carriage is slightly lower than the Taz 5 carriage so you may also need to adjust your Z endstop target.
Great info! Thanks!
You can also make a Taz 6 tool-head and just not put the left cooling fan on it then the fan body would not be hitting the left side stop and height adjust knob. Then you have the bigger barrel cooling fan setup on yours.
But what do I know since all my Taz tool heads are Taz 6 versions. with both fans on them. 
I bought a Ver 2C Toolhead from Amazon and it turned out to be a TAZ 6 design with 2 cooling fans and a larger fan for the Heatsink. It must have been a mistake because the ones they have now are the TAZ 5 design.
Anyway,
I had to glue a 5mm wide spacer to the X Carriage. It works like a charm. You can either buy the adapter harness from Aleph or wire up your own.
I also modified Marlin to lower the Max X value so the carriage doesn’t crash into the right hand side.
Kenny