Due to other commitments I'd lost touch with Fallout 3, so I'm going to start again with (at least) the new Brotherhood of Steel DLC. What I need to know is whether the DLCs change the vanilla assumption that Small Guns is a better choice than Energy Weapons. Are there enough energy weaps in Broken Steel to make Energy Weapons a near-equal option to guns? Also, are the other two DLC packs important from an energy weaps point of view?

I'd say yes.  There are new and interesting energy weapons introduced, and frankly, I thought energy weapons were underrated anyways without the DLC.  Once the (minor spoilers) Enclave start popping up all over the map, there's no shortage of ammo and the weapons do plenty of damage in VATS.  But back on point, yeah, I think the DLC add some variety and flavor to the energy weapons.  I'm still not convinced I want to do a runthrough with Big Weapons yet, but that's slowly changing.

Energy Weapons were always better I thought, even without DLC you got some powerful  energy weapons. Sadly its the lack of variety not the usefulness people look at. They see 3 or 4 guns + unique weapons compared to the dozen/s or so of small guns and so oh yeah I'll go small guns. Not even looking at the fact by end game your drowning in MF and E Cells and the unique energy weapons are some of the most versatile in the game. Hell if you throw in DLC all you need is Operation Anchorage and you'll get the ony gun you'll ever need the Gauss Rifle.