To really answer your question regarding compatibility requires more than just evaluating weldability of aluminum. Concerns over reduced strength and possible adverse effects on corrosion properties after welding must be considered for your application.
Here is another source of welding AL with a very good technical section. All the concerns posted by metengr are covered.
The last time I talked with them they were very helpful with resolving a problem.
Thanks for the info. I had thought about the corrosion and strength issues and will research what effect that will have on our application as it is a structural application. I did find in the Alum Design Manual put out by the Alum Association that 7005 could be welded to 6061 with 5356 filler, but no details beyond that. I guess we will do some testing.