You can use them with tanks, for the most part. A hybrid connector won't work with all tanks, and should probably be avoided for a first mech.
Batteries depend on your build. The lower your ohms are, the more power it will draw. Make sure you are familiar with ohms law, even download an app for calculating the power used. Check out Mooch's battery tests to see what will work for your intended builds. Batteries that support more amp draw will have less mAh, and won't hold a charge as long. But higher mAh batteries will not handle the amp draw of lower ohm builds. Sony VTC4's are a popular battery, they are a 20amp battery that can safely handle 30amps, and have higher mAh ratings than other 30amp batteries. If you don't need the extra amps, Samsung 25R's are a good choice.