Chimney Liner question

Hi everyone, Second question of the day...... The new home that my wife and I purchased needs a chimney liner. In the home inspection report, the inspector noted that the clay lining has almost totally failed and needs to be addressed ASAP. I had a reputable chimney come out to give me an estimate for a stainless steel liner. The owner of the business came out and he looked in the chimney from the basement. He confirmed what the home inspector said, and then spoke about the difference in labor depending on the liner diameter size used. Currently the boiler utilizes a 7 inch exhaust pipe. He can do the job for $300 less if he uses a 6 inch liner as opposed to a 7 inch liner. Evidently if he goes with the 7, he would need to break up the clay liner and remove it from the basement access hole, a tedious job at best. If he went with the 6 inch, the job would be a very simple process and a huge labor saver. Would my series 68 Weil McLain (picture included) operate perfectly fine with the 6 inch liner? The chimney guy said it would be fine, BUT he went on to say, check with an "oil guy". I asked one oil tech ( I am not even sure if he was listing to me), and be bristled with a "yeah that's ok). Well is it really ok? Thanks, Zack