Cutting aluminium - has anyone been told not to laser cut if going to weld?

Don't have a precise answer, my hunch is this is a much more substantial unit. I know it's cutting thickness capacity is much higher, and it's bed size is as well.

 

That is probably the reasons for the increased power usage.

 

I guess the next thing to figure out is whether oxygen has to be totally excluded to prevent oxides forming - or whether the oxygen level just needs to be reduced to say 5% or 8% or whatever. Not sure where we are going to find the answer to that one other than through trial and error.

 

I think that you have not put the issue of possible/minute oxide inclusion into the proper perspective. Thousands or tens of thousands of aluminum boats have been built from plasma cutters and those often using an air shield/air plasma (cut) combination. Certainly with generally accepted welding practices (GAWP) the amount of oxide mixed into a weld are not a concern.
I would be more concerned with GAWP. Starts and stops, crater formation, back cutting to sound material and proper welding sequence to reduce triaxial stresses and distortion are much more important. We have built hundreds if not more aluminum fuel tanks, gas and diesel over the years, with Mig and Tig. Weld failure for us was mainly pinholes that did not get caught
in the pressure test. Almost always it was due to a welder being a bit lazy and not cutting out potential crater cracks at stops and starts.
Ensure that you use the correct filler alloy.