Someone will have to chime in about the saltwater but I think that might be iffy because then you could introduce saltwater into your freshwater fishs digestive system. As for the feeder fish I've seen probably 1000s at once in a 20 maybe 30 gallon tank at the lfs
. I'm not saying this is right. But maybe see if your favorite store will let you see that their water from the feeder fish tank checks out ok and inspect the fish for any telltale signs if disease. If all is good I would personally set up a 10 gallon and after it is cycled buy enough to feed him for a week or 2 and just keep refilling it with fish as needed.