I suppose it depends on what kind of antenna you're building and pieces you're connecting really.
Stainless steel fasteners are usually used for fixings subjected to weather conditions as they are more resistant. However they are galvanically incompatible with aluminium. (Look for bimetal reactions and the periodic table.) As such some antennas use plastic fixings to separate the aluminium from the stainless screws etc. There are less problems with stainless fixings on large areas of aluminium and hence why stainless clips can be used on aluminium tubes and small contact threaded sections. (Tap the aluminium to accept a stainless thread and use a washer at the head end.)
There are plenty of homebrew antennas out there on the internet to get some good ideas of fixing systems.
As for aluminium soldering I can only give you my personal experiences.
Basically, a royal pain in the backside!
I had an aluminium heatsink from a radio restoration that had a broken fin and being an old radio a replacement was unavailable. Seeing as I had the fin I too thought about soldering and ordered some fluxed aluminium welding rods. I decided to try it out first on a different aluminium heatsink before repairing the original and glad I did. Would not solder for love nor money despite videos making it look easy!
The solution in the end was to heat the aluminium up first in a kitchen oven to 200'C before attempting the weld. Worked a treat! I did the same for the restoration piece and it worked out great.
But it'll be a little difficult to stick an antenna in the kitchen oven
I suppose the secret is in heating the aluminium to temperature first but a bit difficult with a torch and large section.
So that said, I'd use stainless bolts and plastic washers myself.
Hope that helps.
All the best,