Games : Where do Harbingers spawn?

I'm trying to get Meat Boy and Bandage Girl achievements and this is driving me nuts.

I know I have to defeat all 4 Harbingers (a.k.a The Horsemen) in one run, but I can't find them. It's not easy to find the Book of Revelations, and each time I get it I find a Double Trouble boss room that ruin my run, or one of the other Horsemen that drop a Pony item (Headless and Conquest).

I've tried the Monster Manual strategy, too, but in 7 hours and 30+ Manual activations I haven't got a damn Cube of Meat at all.

So I'm pretty discouraged, and I've restarted my runs infinite times because of wrong bosses. This is a slow process.

To save a bit of time, I've heard that Harbingers only spawn as the first boss of each set of floors (Basement/Cellar, Caves/Catacombs, Depths/Necropolis and Womb/Utero). So my question is:

Is this true? Or should I try to fight Basement/Cellar II boss before restarting?