Hi. Yes, it's slang, but "half-bucket" is not a fixed expression. Similar, common expressions include "half-baked," "half-assed" and "half-witted," and all of these terms signify "insufficient" in one capacity or another.
I would suppose so. I can only guess what it means as it doesn't seem like a common expression. It probably just is a reinforcer of "small-timer." They are really, two-bit small-timers. Not worth a penny at what they do and meaningless.