Important note for Rock Band downloads from 1.November onwards:

When RB3 gets released on Nov. 1., there will be changes to download titles:

* All songs from 1.11.2010 onwards will NOT be compatible with Rock Band 1,2 and Lego Rock Band
* All songs from 1.11.2010 onwards have Charts for:
- Guitar (Normalmode)
- Bass (Normalmode)
- Drums (Normal and Pro Mode)
- Keyboard (Normal and Pro Mode)
- Vocals (one- to three voices)
* Guitar Pro Mode can be extra purchased for some songs

This probably means, that the 5th gen container will be enough for the RB2 generation. there is a Custom DLC topic on GBAtemp, which explains how to use customs together with official DLC by creating "fake" 6th to 26th Gen containers.