Hey Sarantini,
You can not shoot at a speific individual in a squad with small arms fire. I think it generally hits the closest man at long ranges more often, however not sure.
The only way you can some what target an individual is with area of effects weapons, where sometimes you can opt to attack ground and hit that high unit cluster instead of the individual unit all by himself quite some distance asway. Attack ground isn't 100% accurate though so you can still miss that sweet spot sometimes.
So do you think it's worth it to order your tank, AT gun, or other AOE weapon to attack ground specifically where the flamer engi, or the PGren with the schrek is standing? Or are you better off just attacking the squad normally with the default right-click?