I've been writing an encounter with fire beetles in a sewer. I need a good monster to complement them while keeping it at a level 1 or 2 encounter.
How about a Tainted Ooze? It's a level 1 minion, so it won't blow your encounter budget, and it has a slide effect, so you can use it to group up the PCs for the Fire Beetle burst attacks. It also makes sense in a sewer.
Rats, of course, are the classic sewer monster. You could also think about frogs -- the Thornskin Frog has a knock prone effect, which helps keep PCs in that Fire Beetle burst.
This sounds like a better opportunity for hazards than for monsters.
Putting fire beetles in a sewer immediately suggests all kinds of fun with combustible waste, to me. There could be grates emitting methane fumes, or flammable oil in standing pools or even (dangerously mobile) floating slicks. If a fire beetle or its target is in one of these zones when it attacks, the zone is ignited. The fire beetles won't be much bothered by the phenomenon, but the PCs will have to maneuver both themselves and the beetles to keep the battlefield from becoming more dangerous.
There are some low-level elementals. One option is that the fire-beetles could be living light sources or pets for something like a kobold vermin-handler (level 3 artillery)- who might have a pair of them on leashes or trundle them along the sewer on a wheeled cage of some sort.
Fire beetles themselves have resist fire 10, which means they might share space in a fire-grate or incinerator-pit with a lesser fire elemental (level 1 skirmisher)
I've never liked the MM groupings myself, although that's just an opinion.
I do like Digitaljoel's suggestion on Monster Builder/Compendium (I find them both equally useful as long as I'm not customizing), but wanted to add a bit.
I tend to get a general idea of what kind of monsters I'd like to use to round out the encounter and use filters to get a good list to pick from.
In this case, I'd probably looking at the beast, ooze, and vermin keywords and filter it down to the level range I wanted.
Something I'd never thought of before in my fire beetle encounters just occurred to me, too: it might be cool to pick some creatures that would be drawn to the fire beetles light, like a swarm of tiny bugs.
This is an extremely specific question. In general, the Monster Manual often has groupings of creatures for the entry. If that fails, you can use the Monster Builder (part of the anemic DnD Tools offered to insider members) which will let you search for monsters by terrain type, level, etc. I find the Monster Builder much more valuable (and usable) for planning encounters than the Encounter Builder flash thing they have online.