No but 10 will. It's having more full ranks than the enemy. The Steg counts as being a single rank because monsters stand alone.
There is no such rule for Monsters. 5 Skinks DO get Steadfast against a Stegadon since they have a rank and the Steg doesn't.
To be honest, your right - I can't find a specific rule that states that monsters don't have a rank but there is also no specific rule which states that a monster does not have a rank. This will be something to get FAQ'd. I believe that a monster is a rank because it can only be on it's own therefore making it a 'rank' of fighters, it can simply never get more than one rank. Ondjage: this is for you to decide yourself amongst the guys you play with.
Actually the rules for what is and what isn't a rank are very clear. No single model qualifies, monster or otherwise.
On page 181 in the mini-version off the Warhammer rulebook, in the troop type overview box, it says "n/a" in the monster rank width slot, so I guess they don't have ranks?
I also play woodelves and I can fully attest that five regular models or 3 monstrous inf/cav constitute a rank for steadfast since it means having to grind a unit down to nothingness in order to break them. It is so bad that I often throw archers into combats on a flank so that winning by 10+ means something! The issue of whether or not one counts the front rank is often missed since it is contained in the italics example given under the steadfast rules on page 55. However, once you read the example, it is clear that the front rank is counted for steadfast.
The newest FAQ confirmes specifically that 5 units are steadfast against anything with no ranks (linke monsters). The FIRST rank (those 5 models) is enough