I wanted to do double check that it is okay to soak the populated legato, placid and lcdps power supply in a tub of isopropyl alcohol for a bit before cleaning with a toothbrush. I don't think any of the components would be harmed but want to check. Thanks.

Most components are fine in isopropyl, but I wouldn't let them "soak" because I worry about things like electrolytic capacitors. Even though they are "sealed" the volatility of alcohol is such that I wouldn't want to submerse them for a long period (you're just asking for trouble.) The best method for me is to pour the alcohol over the board so it gets nice and wet, scrub it with a brush, and then rinse it again. No need to test your luck :)

Using isopropyl alcohol you don't need to soak the board to get the residue off. Just sprinkle a tiny bit on the board while keeping it upside down. Then use an old toothbrush to clean it. I generally use the special spraycans that already have a brush attached to the nozzle.