It is worth pointing out that the Rock Band games (the first of which was released in 2007) added vocals and drums (which had four pads) before the Guitar Hero series ("Guitar Hero: World Tour" was the first in that franchise to add voice and drums, and that did not come out until 2008, and they added the extra pad for that particular franchise, although the GH series also had a mode containing just four pads for those people that already had a Rock Band drumkit and did not want to shell out again). This is why the drums only have four colours in the "Rock Band" games.
In terms of downloadable content that is compatible with "Lego Rock Band", the Rock Band DLC that was released before Rock Band 3 came out in 2010 will be playable in it as long as the song is deemed family-friendly. Even the family-friendly songs from the first "Rock Band" game that were exportable from that game could be played in "Lego Rock Band".
One downside nowadays (at least, for those who have other titles in the "Rock Band" series) is that, while it was once possible to export all 45 songs from the XBox 360 and PS3 versions of "Lego Rock Band" to the core titles in the "Rock Band" series, now that this game has passed its fifth anniversary (it came out in November 2009), the licence that allowed that particular export has now expired and so it is no longer possible to export the songs.