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.
It's quite derogatory and yes, it is slang. Bucket can refer to women's genital parts or to mental retardedness so whatever meaning it is here it isn't exactly being nice. Small timers are losers.