I used the same material for my array racks. 1/8" angle iron, though mine was 1 1/2". I have probably 11 of these racks now, due to my adding slowly to the array over a 25-yr. period. And, I have to wrestle them by myself up a steep hillside, so smaller, three- or 4-panel racks are what I did.
I left an eighth inch between panels. I don't know whether I need any space at all, but I did it that way. I have never measured the ambient temp' up at the racks, but our temp' up here can be -20 F. to +98 F. or so. I made the fastener holes slightly oversized for expansion/contraction. I have seen no sign of stress in even the oldest panel frames.
I cleaned the angle iron well and painted with regular metal paint, sometimes from spray cans and sometimes brushed on, depending on what I happened to have around. I did nothing else to prevent galvanic corrosion between the aluminum PV frames and the steel rack. So far, since 1985, I can detect no corrosion. If there is any, it is so slight I cannot see it.
Now, I live in a very dry area and environment. It is not unusual for us to see 10% humidity or less. I cannot attest to whether my minimal method would work in a humid area. If you feel you need to isolate the aluminum frames from the rack material, you could use any number of non-metalic materials you have around, for no money. Plastic, etc. Obviously, if your array will be anywhere near an ocean environment, you must be careful to isolate all dissimilar metals.
I did drill some weep holes in the lower horizontal member of each rack so that rain water/snow melt could depart.
