In all the districts I've ever worked in a position where I had to give a **** about them, and all the ones I've installed and worked on over my time in the two trades, it is not legal for any private entity other than contracted fire crews to use fire hydrants. The penalty if caught by someone who cared was a warning, then $500 fine for the second offense, followed by larger fines, and they can be given out by the acting company commander of a single resource (engine boss, truck Lt./Captain.) It is not a felony in my area. It's not like we have our engine crews out patrolling either. It's pretty rare to see it heavily enforced unless there's a severe drought. I'll usually just ask people to stop. And most of the time people do.
Something else to consider is if the hydrant is private instead of public. You can get charged with theft on a private hydrant. And there's usually nothing to differentiate public and private hydrants.
However... If you're putting out a fire with your rig supplied by water from a public hydrant, chances are it'll get labelled good samaritan and no one will care. You may get a bill if you use a private hydrant.
One last thing about house fires... Chances are, one untrained man with a skid unit and no proper PPE with modern construction and fire loads is looking to killed by any multitude of ways in a hurry. There's a technique called SLICE-RS (a structural tactic developed for 3 man engine crews on buildings <1500 square feet per floor max 3 floors) that I suggest looking into if you're dead set on trying yourself, but just please don't. Just wait.
