You can place songs in folders in DTXMania. Just name the folders with the following prefix so that the system recognizes them as folders of songs rather than song folders:
DTXFiles.xxxxx with the x's as whatever you want the folder's displayed name in-game to be.
If you have the HDD space, I'd recommend you also make a few folders with songs of similar difficulty because the game also lets you do Random Song by folder as well.
If you haven't played one of the more recent DM AC mixes, the games offer these sortings:
Default (not exactly sure how they come up with the default, but it always starts with newer songs and works down the list into older mixes, however it seems like certain series of songs and harder songs end up higher on the list.
Alphabetical: B follows A, C follows B, I really hope this one was self-explanatory.
Difficulty Level: Sorts by Basic, Advanced, and Extreme.
Difficulty #: Sorts by numerical difficulty.
Release Mix: Usually what my DTXMania is defaulted into.
Long Songs Only: Self-explanatory
Random Songs: Offers different ways of choosing random songs, such as random long songs only, random regular songs only, random new songs only, and random existing songs only.
Secret: Sorts by songs that are unlockable on that mix. On DTXMania, you could just stick in the e-Amuse songs if you didn't want to give them their own category in Release Mix sorting like the AC does. Encore songs could go here too, I guess.