Hello skye,
We (several of the companies I consult with/for) average a silo call every few months for sale or restoration. I will do my best at expanding your questions if I miss anything...
What sort of foundations did theses have?
Originally stone, cob/lime, and timber, and in the vernacular forms of silo, granary, and the related "dry food storage" structures, this is still the most common practice...
Does anyone know if a (wrapped in landscape fabric? I guess, though i don't like the material) rubble foundation would work?
It could...if designed a built correctly.
I can suggest, If you don't care for the "landscape fabric" or "geo cloth" (though some are post consumer recycled materials if you look for them) these fabrics are not a "must have," in more vernacular design...
Regards,
j