Okay, it seems that there is a bug in the game. I don't know it it was reported yet, but when you change one type of unit into another in scenario editor, the size remains the same.
It apperas that those snipers were the trucks first (which original size of 3.)
Also, there is another bug - game doesn't keep some the modified core unit data in the campaign. First mission is fine, but after it game acts weird
. For example it resets ammo numbers, and if weapon was deleted (not replaced!) the game restores it.