I use Maxi-Jet 900s to mix sw and move it from the bucket to the sump (one per bucket). I can definitely see how a MJ 1200 would be too strong for a 5 gallon bucket, as I use the 5 gallon buckets the salt mix comes in to mix sw and the MJ 900 really churns and splashes if it flips the wrong way. BTW, I don't attach the powerhead to the bucket, it just sits in the bottom of the bucket. However, the magnetic mounts made for the MJs by AlgaeFree are strong enough to hold a MJ to the side of a standard 5 gallon bucket despite it being a rounded surface. The Hydor Koralia magnetic mounts are as well, at least on the #3 and #4 models, as I've done it while cleaning those powerheads in a vinegar solution.
As for a single pump to circulate water in numerous buckets all at once, it could be done if you really wanted to. A little bit of imaginative plumbing and pretty much any return pump that'll move water from a sump against 4' of head pressure to a display tank with good flow would be plenty strong enough to circulate the water in multiple buckets. I see a couple different set-ups in my mind, but I'm not really sure they would be worth the expense or time to construct considering how easy it is to just drop a couple MJs into buckets.