Improved DIY Tensahedron Stand:

[jeff-oh]

I'd be concerned about steel and aluminum in contact with one another, especially outdoors b/c of electrolysis (besides the potential strength differential)

I do not have any of these concerns.
From: the American Galvanizers Association:
Under atmospheric conditions of moderate to mild humidity, contact between a galvanized surface and aluminum is unlikely to cause substantial incremental corrosion.

Your not going to get a corrosion issue due to galvanic reaction (electrolysis) in overnight use.
If you still have a concern then you can always apply a protective coating. (paint)
There is no issue with differential Yeild stress of materials (strength). Just have to make sure the weaker material is capable of the task it has to perform.
But, yes, I would agree, if you are going to purchase tubing, I'd purchase DOM steel over AL. However, if you have AL available use it and do not worry.

[P-Dub]