I picked up a dual coil tank just to test out, will it work with my ego-c at all or should I put it aside for when I decide to get a vv?
With the batteries, I treat them how I treat a laptop. Don't let it drain, don't leave it plugged in. You'll get a "feel" for when the batteries NEED charging. Vapour production (and taste) drops off dramatically for the last few pulls. I'm not sure of your exact model, but every battery I've seen is Li-On so they shouldn't have any of the old "charge memory" issues of yore.
I'm no expert on knockoff matching with official ego, but I'm assuming that it's all about the threading. If it screws on you should be good, leaking will probably be coming from any seals on carto/clearo/tank. Trying to force a part with different threads on can potentially cause some physical damage to the threading (same as wrong nuts on bolts). If you have an automatic battery you will have to be extra vigilant about not getting fluid on the battery connectors, word on the street is juice can gum up a perfectly good battery (haven't had it happen to me yet xfingers).
If you see a part you like, make sure the threading matches or get an adapter. Ego's are 510. Don't quote me on that though, I'm still new and am still trying to figure out how to get my drip tip connected to the atomizer...lol