Okay! So on the other forum, there was a 400 PM message limit, so I increased this one to 500. :3 So that should free up space!
I can't find an option for folders, but I do see a way to sort mail. If you go into your PM settings, you can set "rules" for how you want your mail sorted, and also apply labels to mail. I haven't played with it much yet, but it looks like it'd be a good way to organize PMs that may function similarly to the folders. :3
Oh! And if you go into your PM settings, you can choose to display your PMs as conversations. :3 So that'll set your inbox up in a fashion similar to the old forum's PM system, where your PMs read like a thread (with your replies included) instead of them being thrown into this giant pile of chaos! xD