4) Since the Training folder is purged by SmarterMail, and not by antispam products, multiple products can learn from the Training folder at the same time.   It is a not a pick-only-one situation.

1) #1 should still be possible you'll just miss out on the per-user stuff and user's ability to hit the dashboard or configure rules. This is something we may meet about, the additional ways it can be used in SM or how it could sync across gateways.

2) Seems like a valid approach. Also If you want to control the cpu "workload" of SpamFoo it has a concurrency setting in the dashboard that can be configured to control how many threads/messages get processed at once. 
3) You can use spamfoo on free smartermail during this BETA period. So you can easily just install or run a SmarterMail docker for example and immediately turn on spamfoo and use it.
4) SpamFoo doesn't use the training folder for its training-loop, but you can still utilize it concurrently.