putting them in cold water slows down their metabolism ( ie they don't need to be fed and they need high oxygenated water to thrive/survive. I have a very small colony already in my sump of worms that got sucked up and they help eat any bits and pieces.) But they normally are found in warm highly oxygenated water. probably why the ones in my sump are chunky and by all appearances thriving there.
as for piggys... YES! It's amazing to see how much juvi fish can and will eat. this is also why my retics probably have 10" disks at 2yrs old.
And yes I've seen what 2lbs of blackworms look like. lol it is impressive but as OP stated fish can and will consume much larger amounts of live vs pellets ime. and why I suggested not dumping 2lbs in right off the bat but seeing how much your filtration can handle.
Live foods ime require looking at as "stock" making adjustments w/ water changes and biological filtration. I imagine as well once you get a nice amount started you can add x amount per week to keep the colony stable.
It's just as much "work" as taking worms out and rinsing them daily... ect but its different mainteneces ect.. I "prefer" to give my juvi fish (1-2yrs old average species ect) free choice in live foods and supplement with pellets ect. Then try and monitor every feeding ( some juvies should be fed 3-5 times a day, I kept sw pipe fish years back and hated having to feed like that) Some people don't agree with my husbandry practices, which is fine. In the end we all do what we think is best for our fish. That being said as well I also treat for IP's every 4-6 months to inhibit parasite infections ect.. most fish keepers don't... So my routines ect aren't as cut and paste as they may appear. Even though I qt/rinse and otherwise take precautions feeding live it is just as much a risk imo to feed a processed diet exclusively or frozen ( vitamin stripped) diet... not to mention all the things we don't know yet about their feeding requirements.
Just like the rest of the aspects of keeping "higher level fish" we all have our "methods of madness" .. I over filter and do copious amounts of water changes... and while I keep a imo low stock level my live foods take the place of other stock ect... So in an overstocked, adequately filtered set-up keeping live blackworms may have a more negative impact on the whole system and therefor your pets... at the end of the day we are trying to replicate nature... Good luck to us my friends!