Warlocks can basically play three different styles.
If you go Demo, you're essentially a BM Hunter.
In this case, five locks would be as good as four.
Glyph and talent Health Funnel, and use that + Life Drain for pet heals.
You could go any healer quite well.

If you go Destro, you're basically mages who want to nuke things down.
Again just about any healer would work, since the Locks will Life Tap for mana.
And then the healer can heal away.
A Resto shammy might be an idea, as Mana Tide helps the whole group.
And Healing Stream totem is passive healing for the Life Taps.

If you go Affliction, you are primarily a DOT team.
Sure Haunt and Unstable Affliction have cast times.
But you'd primarily Agony/Corruption on things, not even stopping.
Run to the next mob and Agony/Corruption it too.
Usually play with 5-10 mobs wanting to kill your team.
Have Howl of Terror on round-robin.
So a mobile healer would be ideal; Druid or Priest comes to mind.
The priest will get AoE fear (Psychic Scream) quite a bit before the Warlocks can talent Howl to be instant.

5x Rogues would level well.
Not sure it would be that playable at higher levels.
Sure you could gank in cities, with great stealth.
And re-stealth somewhat reliably, even in combat.
Aside from the re-stealth capability, I'd say Feral Cats are superior to box.
Both have strong DPS, but Ferals can heal/rez/battlerez, which puts them over the top.