does it make sense for an enterprise to create supply contracts with its corps and order supplies for it so it is possible to control individual supply quality's or is that just to conplicated
Yes, do it. Set up contracts between the corps. You need to work out the average of all your corps supplies quality though, remembering its a weighted average.
Nope, can't alter downgrading rate if you mean the quality and effectivity upgrades. Sure you can downgrade quality quick, sell what supplies the corp has and replace with lower quality or buy a load of low quality to dilute what it has already. You can have up to 60 months worth of any supplies.