RE: Minimum globally routable address space?

If you're going to get a /20, I'd say get your own from ARIN. The biggest
pain of a routing policy I've had to deal with (as far as other people's
policies go) is Verio, who only accepts /20 and greater for all of the
recently opened legacy class A networks (64., 65., 66 etc)

As long as you don't try to slice and dice your /20's into more specific
blocks to come out of other places, you'll be fine.