On some G&G's their crane stocks can get a little tight fitting on batteries over 18 mm in diameter
some nimah's are a bit thicker and the external wire might not wish to play ball when trying to fit it in there
some stocks are fine - but did notice one or two various stocks are more roomy than others is all I'm saying
normally 18 x 18 x 125 length is a rough guide - some crane stocks are a little more roomy than others
but be careful if your battery dimensions exceed 19mm & doubt if 20mm width ones will fit in there easily
yup you can go nimah 8.4v or 9.6v
or go lipo
http://www.componentshop.co.uk/7-4v-2000mah-20c-continuous-discharge-airsoft-lipo-split-pack.html
http://www.componentshop.co.uk/700ma-charger-for-1s-to-3s-3-7-7-4-11-1v-li-po-and-life-battery-packs.html
now that is a fail safe battery that will fit easily and a basic no fuss - slightly slow but simple to use charger
best chargers are the Imax (genuine not fakes) B6 type chargers but adapters & psu plus getting the settings entered for you battery
(nothing major to get your head around but the B6 will charge any battery you may ever purchase - good investment if you can run to it)
there are cheaper lipo's especially if you shop at RC places like hobbyking - cheap as chips but you need to work out if any battery you buy will fit
(crazy prices but most are block type lipo's and stuff like changing connectors etc.... - they do some airsoft batteries but check the sizes yourself)
you need a small or mini tamiya connector for your bog standard AEG as that is what they come fitted out of box
nimah's are fine but you will always pay through the nose at airsoft retailers
nimah's are old school but 9.6v provides plenty of snap - though a good modern 7.4v is not far behind it and really holds more grunt in general
any charging should be done in a safe area, constantly monitored - not on the carpet
say a garage, utility room, not your best dinning table - common sense stuff
a lipo charging bag is a wise consideration - but if you are sensible they are safer than a Samsung Note 7
if you are very very stupid though......
it is video I post a lot I know but though this is extreme it clearly sends out a message of respect & how much energy they can release
(don't be stupid - I have a scorched hall carpet still from being a dick and thinking I'll let a bit of air out from a puffy lipo - if they puff up then replace)