Firstly is it advisable to keep a larger cylinder at home to refill the 2.5 ounce rifle cylinder.
Yes. You can rent them and keep exchanging for filled cylinders when the one you have runs out of gas.
2)What r the adapters etc required to accomplish this task?
Take the rifle cylinder to the place you are going to rent the cylinder from. He will either have the adaptor or tell you which one to buy from the hardware market. If it's not readily available in the market you can have one made in brass or you can import one provided you know the specifications.
3)What is the pressure that is usually used to fill 2.5 ounce cylinders?
CO2 cylinders have to be weighed after filling. So rather than a manometer you will need to invest in digital or simple jewelers weighing scale. Read your owner's manual to find out what the filled cylinder should weigh. Before filling, put the cylinder into the fridge for about 10 minutes, then immediately attach it to the main tank and fill at a slowish pace and making sure that the airguns cylinder does not turn too warm. Once filled, weigh the cylinder and if necessary you will need to remove some gas to get to the correct weight. A discharge valve may be enclosed with your gun.
In case you need to fill the cylinder at the range where you may not have access to a fridge, discharge the gas from the cylinder quickly and it will turn cold and will be ready for filling.
CO2 will normally have a pressure of 800 psi when filled into your cylinder, so be careful and do not be tempted to use any other gas besides CO2.