Ego can use any atomizers and cartomizers of slim 510 since 510 uses standard resistance atomizers and cartomizers.
The reverse is not true. The Ego can also use low resistance (LR, low ohms) atomizers and cartomizers which are hotter and draw more power. Slim ecigs cannot use LR stuff - it can damage the slim batteries. Similarly, slim 510 cannot use the Ego's charger cable - it puts power in too fast and can damage the 510 battery - I would label the Ego charger cable so you don't screw the 510 battery into it.
In addition they can both use Joye 306 atomizers (slim 510 can only use the standard resistance ones, Ego can use standard or LR). Joye 306 atomizers need 306 drip tip or carts (but don't drip on auto batteries if you have auto 510)