I recommend manual batteries which are much harder to damage or kill with juice leaks.
See KR808D-1 manuals if you want prefilled cartomizers you can top off with more juice to get at least several days of good use out of each - Vapor4Life, SmokelessImage Volt, Bloogplanet and I only recommend manuals because when a cartomizer leaks it will leak right over where an auto battery can be damaged, Manuals also make it easier to get a good hit.
Or if you are willing to fill blank cartomizers, I recommend Joye 510 manual w/ Boge standard resistance blank cartomizers (or the long version, Joye 510 mega xl) Not Joye 510-T - if you want to try tank atty and tanks you can put them on a normal 510 battery.
Those are the only two slim ecig models I recommend. The smaller sizes of each are about 100mm long with cartomizer. The larger 280-320mah KR808D-1 is about 120mm long w/ a carto, the Joye 510 mega xl is 128mm long w/ a carto.
Cartos are just for juices you know you like and I find thin clear juices work best in my cartos. For thicker juices and testing juices I use an atomizer and a drip tip (drip tips are only recommended for use on manual batteries). 510 comes with atomizer. A DSE901 atomizer will usually fit on KR808D-1.
Slim ecigs must use standard resistance atomizers and cartomizers. If you want to use the hotter low resistance parts then you has to get a big ecig - fat battery or mod, with 450mah of charge or more.