we converted our garage to a bedroom,when we had the furnace replaced the hvac contractor said we could use 2 7" pipes to supply the room.the room is 272sqft,so i was going to use a 6" and 7" round feeds.can i use a single 8" return or should i use a "T" off of a 10" pipe the run my 6&7" pipes?

I simply love it when someone changes what the contractor said to use. Why are you not using two 7" like he said?
First off.
Did anyone do a calculation on what is needed?
Just because its only 272 square feet. Does not mean that the two ducts will have enough supply to properly heat or cool. My guess not. remember you have a heat loss on all sides. Heat gain in summer. Is the attic properly insulated? How much. What color is the roof. How much overhang over the windows, what direction is the house facing? Way to many questions.
My suggestion is to have the hvac contractor do the job. This way when it does not work, You can go back to them and have them corrct it on their dime. If you do it yourself, and it does not work. You will end up doing it over and over again until you run out of cash. The reason is..

A 10 round duct can only handle about 250 CFM of air flow. Sure you can push more through but its noisy and not suggested.
A 7" duct handles @ 180 CFM max based on the duct friction of .10. So the total amount that would be needed to supply both of these would be 360 CFM. Your already short by a 110 CFM. But the supply will not provide that. However if this room is going to be a bedroom. He may have suggested using this size duct to slow the air velocity down so when it blows out of the grills its not as noisy being a bedroom and all.
But is the room requirements 250 CFM of air?

Also your question is sort of blurry. Your asking if you can use a 8" return? Are you now talking about the return duct back to the furnace? If so, no. You need to return as much as you supply back to the furnace. Otherwise it will starve on air and the flow will suffer. Some theorys say its ok to be a little short as the room will pressurize and air from outside will not enter. That may have been true many years ago. But houses are built much tighter and ducts in most areas must be sealed on all joints so the furnace no longer can get air from the leaking joints as it once had.
Also is the 10 pipe already sized for another area of the house and you are simply tapping into it or is it run just for this room?

If its being used by another room and your simply tapping into it. You will have a air flow loss to a point that the room over the garage will not heat or cool and the same will end up at the rooms where you tapped the air from.
Run your own supply from the main plenum on the furnace. Do not tap into the supply trunk unless it was sized to add any additional ducts to it. Otherwise you will have issues with air balance in the home.
Also use metal pipes, And do not tap into any flex ducts if that is what was planned. Although it will seem to work. The loss at the connection points are so bad that your comfort will suffer elsewere as well as youe pocketbook when you end up spending more on energy and still more on the service calls needed to fix it.