Washdown pumps, keep in mind they are typically fairly low flow and most are not really made to run continuously. Some are better than others in this regard. If your pump fails after a couple days of this duty, consider a dedicated livewell pump. I'd say most washdown pumps aren't great in this regard.
As far as the low flow aspect, if you place your drain fairly low on the barrel, ie so maybe you get 20ish gallons and not actually nearly 50, that should be OK. You will know if there is not enough flow because basically the bait just doesn't live long. A fairly full drum doesn't work with typical washdown pumps, bait doesn't last very well.
A 1.5 inch drain is adequate for small pumps, but I'd stick with that size and not smaller because it's common and easily available ($15 plastic through hulls are available versus say a 2 or 3 inch fitting that brings you into fittings that aren't cheap). However as you go to larger pumps they can require larger or multiple such drains - last time I setup a barrel it needed 3x 1.5 inch drains because of the pump size, otherwise just filled up to top and constantly dumped out partly onto deck.
I mention this because we are talking temporary livewells, cheaply done etc etc. Do not turn your back on a running livewell unless you've got the boat and whole system done right and figured out, ie leave it running at the dock or while you sleep. Dumb things happen, ie inlet or outlet hose fall off or end up out of place causing water to drain and/or be pumped into the boat. Bait dies and clogs outlet causing barrel to overflow onto deck continuously and some problem on the deck allows that water to get into bilge. Etc etc.