Is there any way to guide what class a rookie will become?

According to readings around the internet - no official sources - it's random. In my experience it has been random. I think I've gotten 2 heavies, 3 snipers, 3 support, and 2 assaults (still have a few rookies yet to get tested out). How many guys have you seen promoted?

Note that I play rookies exactly one way: move closer to enemies. Draw fire. Go through doors first. Shoot or capture if possible. Pat on back if they survive the mission. ;)

You could try saving your game before ending a mission, and then reloading to get a different promotion.

I got this idea after a PC crash. I loaded and repeated the last action, and 2 rookies became an assault and sniper instead of the heavy and heavy they were previously.

They are completely random.

I got 3 sniper once and in the next mission 3 supports... I think that my game just like to troll me...

From wiki:

Rookie

A Rookie is a fresh Soldier who lacks a class. After going out onto missions and gaining a small amount of experience (approximately one kills worth), they receive the Squaddie rank and are randomly given one of the four classes: Assault, Heavy, Sniper, or Support

Assault

Rookie promotion is slightly weighted toward classes you have less of, but promotion is otherwise completely random.

Heavy

Rookie promotion is slightly weighted toward classes you have less of, but promotion is otherwise completely random.

Sniper

Rookie promotion is slightly weighted toward classes you have less of, but promotion is otherwise completely random.

Support

Rookie promotion is slightly weighted toward classes you have less of, but promotion is otherwise completely random.

Psionic

Soldiers belonging to any of the principle four classes can access the abilities of this sub-class in addition to the abilities they already own, however these psi-abilities only unlock for soldiers who possess "The Gift" which is determined by testing in the Psionic Labs.