If you got the Rubbermaid BRUTE 44g trash can, yes that's the one (I have 2). You can siphon from wherever you feel appropriate, sucking up waste is a good idea if your filter isn't taking it out. I usually siphon any loose waste out of a sump every couple months.
As far as aging the saltwater mix, you can 'technically' use it after about an hour (the label for Instant Ocean says "as soon as salt is fully dissolved") but I usually let mine mix (and aerate) overnight. Usually. This is more important in Reef systems, IMO
. I find that the salinity usually stabilizes after a few hours, but I always like to wait longer to let everything FULLY dissolve and heat, then test salinity in the tank first, then the new water so I know it's closely matched (using a refractometer).
You will likely have to turn off your return pump, because when the DT level drops below the overflow line (depending on how much water you change) your return pump will start sucking air and that's bad. You can probably leave the skimmer running, it just won't skim when the sump water level drops down, but will return to normal operation when you re-fill the tank (and sump). I have a couple tanks in which I purposely do not unplug the skimmer during a PWC
as I have found that they do not balance out right away after plugging in again - they overflow for about an hour, then settle down. Your may be OK with a power on/off, so just try it and find out (better to find out on purpose than by accident, like during an outage!!)
Edit: sorry for the repeated into...stepped away for a bit