First of all bro I am not at all an expert. lolz. just as an owener of vitz I have heard these questions a lot at pakwheels and other forums. Mortal bhai has already answered the questions well.
1) The best tyre size in 15 inch is 185 60 R15. As I said swift tires will also look very good. 195 60 R15 is no doubt a large tire but gets handsomely fit in the car. With full load there are chances that the tire will touch the body when suspension springs squeeze (contracts) to its full extent. But it overall works fine. My tires touch body sometime when I touch sudden trough on road at high speed. You can go for 185 60 R15 if u feel uncomfortable for your suspension. It is OEM size
2) Intakes simply do nothing other than flowing the air into engine by force. It provides better burning of fuel due to better availability of oxygen. I have not used it. In theory it should give u a better pick. Practically I have no idea. You will have to ask other experts.
3) CNG!!!!!!!!!!!!!!!!!!!! Read mortal bros answer ....... read that again and again .... never ever think of it. This model of vitz has CVTi transmission. CNG means death!
Best of luck