I need help with my ICS CXP 16-S
 

So I recently bought an ICS CXP 16 (short, metal version) and with the package, he would give me 2 batteries (8.4 V 1100 mAh) and 2 mags and etc..

Okay so, The battery that it came with just wont fit in the buttstock even if I fully extend it. It logically just won't work. I tried contacting the guy about this but he hasn't answered. I was wondering is it because the battery is not for the gun? I tried fitting my 9.6V nunchucks in, and it fits perfectly fine (connected them together but never fired a shot because I have no idea about AEG batteries.). I just don't have any idea if the one in the package is the right battery to use or not so I just assumed that it is the battery that I'd be using.

Another question: Is it okay for me to use my 9.6V 1600mAh Nimh batteries to my ICS? and would you recommend it? Are batteries universal to each AEG around or at least most?

Batteries are pretty generic. Some newer guns like the G&G with ETU's require 11.1v lipo batteries to work properly.
But your ICS will work fine with a 7.4 lipo or 9.6 nimh battery.
There a sticky post on batteries if you want to know more info about them.

Is it better to use 8.4V rather than the 9.6V? Thank you for the advice!

Personal preference. A lot of people like 11.1v lipos.

9.6 in the 2/3A sized cells will be fine

http://airsoftcanada.com/showthread.php?t=156571