Brushes: read the labels. They'll tell you what you can use them for. Do not deviate from that, unless you're just using them to apply stripper in which case go to the dollar store because you'll be throwing them out. Possibly in pieces, which is amusing.
Also, please stop taking everything the OSH employees say as gospel. They're just another hardware store, and no more likely to know what they're talking about. If you're going to believe any employees, they need to be trained/have experience. If you find a person who does have knowledge/experience, awesome, but don't assume they do. Doesn't mean you can't ask, but still do your research.
Personally, I use Sherwin Williams. I go to their store (not Lowes, though you can get paint chips/samples from Lowes and the the standalone store can use that info. Lowe's doesn't have the really high quality SW paints). Those employees either HAVE been trained, or have had enough exposure to people who know what they're doing that I can believe them. I still double check what they say, but they've never been wrong.
In general, the prep work is THE most important part of any painting project. I wouldn't skip the TSP. If you do, and you really did need to do it, then you may end up starting over at some point.