Its hard to say if the gas carrier grenade will fit in that pouch. The couple I have seen and handled in my day are maybe only about 1/4 of an inch thicker than a Redbull can and essentially about the same height if only maybe a 1/4 of an inch longer as well.
They come in all shapes and sizes. But be advised there have been comments about the small amount of gas they carry, or its also been noted that these items are sometimes prone to leak, but with good maintenance and love, just like taking care of a GBB mag, you should be alright.
For refrence of diffrent types go to:
http://www.wgcshop.com/
And simply type the words : gas charger , in the search bar.
You will see 3 kinds of portable gas chargers.
As per your main inquirey I can offer no insight into whether or not you can use other forms of gas.