Can the salamander breathe weapon be fired once per game in close combat? or can do it as maney times as I want?
Salamander's weapon is not a breath weapon it is a templated shooting attack. Can only be used to shoot, so no effect in combat and cannot be deliberately used against a combat. "Accidentally" shooting into combat is another question. Can also march and shoot. Skink handlers cannot be targetted for either shooting or combat, cannot be used to trace LOS and are not eligible to be contacted or used as point of measurement for charges, cannot block normal movement and generally do not exist apart from making close combat attacks and acting as a casualty sink for the unit.
Now this has come up in a game before, where I could have done it but didn't. Due to a failed charge and some weird positioning an enemy unit was 1" away from my Sally. I could have tried to shoot this unit but there was a unit of Saurus and Ghouls in CC behind it. The template would have overlapped the CC no matter what I rolled. Now how would you have resolved the wounds on the unit in CC? If you read the Skaven Codex they have a method for resolving shooting into CC since they have units that can however it didn't seem like it was the intent of the rules to allow for the Sally to do it under any circumstance, so I didn't fire. What would you have done?
I have ran into that situation a couple of times. When my sallies template went the full length (10 inches) and hit CC or another friendly unit (chameleons). That is the only time that I would do it. If it is a shot that would be almost an autohit on my a friendly unit, I would not take the shot. If the template would hit a unit in combat (with a large chance of missing), I will take the shot.
Since the salamander's attack is a template, there is no need for any special rules for hit allocation like Skaven have. Each model under the template takes a hit, regardless of which side they are on. So it is very possible to kill your own models as well as the enemy.