is what buider says true?

My sister recently had a new heating system installed,she also wanted to move the cooker gas pipe position in her kitchen ,having got a plumber to do the work another builder said that her heating would not work properley as the plumber had reduced the supply to the cooker from 22mm pipe to 15mm pipe ,theres around a foot of 15mm pipe to supply the cooker,she was hoping to install an eight ring cooker,but he said heating would not work properly if all burners were lit at the same time when heating was on.Is this true or does the gas flow under pressure as needed.thanx