Can I have a different company fill my home heating propane tank?

Sure you can but if the company that owns it were to check it and know you filled from a different company they will pick up / remove the tank with the propane and legally do so because when you are furnished a tank from a propane company you are not to break the contract by buying gas from another company. If there isn't enough gas in the tank to matter, I'd have it picked up and go with another company. We have ours filled off season in the summer and it's cheap in the summer time.