Milk, If you can read the book, I'd recommend it. It's much better for those of us who thrive on initial flurries than other systems. It's all about the initial flurry! And there are bits that clarify some of your questions.
Keeping clothes with memories attached... Not so much. You "thank" them and get rid of them. To me, that means really acknowledging why I was keeping them, and taking a moment to consider whether I'm keeping them for me as I am now, or the person I once was. If the latter, they're out - I don't have the space to store stuff for people who no longer exist!
Similarly, with stuff you're storing for some future version of yourself (pregnant/working/thinner... We all do it for different reasons). This is a balance we each have to find for ourselves. When I went through my thin clothes, I asked myself whether this was an imminent ambition (not likely), whether it brought me joy to be surrounded by a thin person's clothes (definitely not!), and whether any of these items would really bring me joy if I fitted into them right now (actually, no!) If you can answer yes to two or three of those questions, then you should treat your maternity clothes like everything else... Still reduce it to only the pieces that really bring you joy.
As for practical versus joyful, there've been a few discussions upthread about the joy of things that just work well. Like my joy when I put on a pair of perfectly dull socks that are smooth, non-holey, fit well, and don't fall down inside my boots. The plain black t-shirt that you pull out of the laundry and think "Yes! My favourite!" You only really get the sense of it when you start going through them. But if you have a perfectly good pair of jeans you wear a lot because they're new and they fit, but every time you see yourself wearing them you feel like a potato, they have to go, no matter how practical they are!