No harm in getting CNG in your Altis, but remember it is a dying fuel, shortages have already started showing their ugly head this winter so hard, even there is no gas for cooking and heating at homes. I fear that in next winters we will have complete CNG holiday and in summers too loadshedding of natural gas will start.
I think if you want to keep the car in warranty get it installed from your authorised 3S dealer. If warranty is not important for you go to "Compressed Gas international workshop, Plot# 93-E, St 7, I-01/3 (051-4445130), It is behind the CNG station in front of Suzuki Motors showroom. They have original stuff and the mech Salman Shah his the best. I installed CNG (original Landi kit and Landi emulator and advancer) in March 2008 in my Xli and Al-hamdolillah have never faced a single problem since then and the pick is just like if not equal to petrol.
But keep in mind (also all those planning to get CNG installed in cars)
Most of our industry when faced with electric loadshedding installed their own power plants and now running those power plants on natural gas, which has caused tremendous shortage of gas. Now even in summer when there will be huge electricity loadhsedding, more and more gas fired private power plants, generators will be installed all over the country.