You chose a large bulky one, but its a nice one and similar ones are used by companies like Lacroix and Kaly. (I believe) You can use a GX16-3, a GX12-3 or a simple 5.5x2.5 barrel jack, but it just depends on how fast you charge. There are some fully metal barrel jacks that can apparently handle 10 amps without heating up too much, and its a fairly nice and small connector.
A focbox unity has a power switch built-in. As long as the circuit for that power switch (called the antispark) is working, you don’t need one. They tend to fail a lot, so worst case you either replace the ESC with something like a BKB Xenith v2, or a stormcore 60D+ (which have antisparks), or an esc that requires an external power switch/loop key. A loop key is essentially a dumb device that allows you to turn your board on without sparks. The XT90-S connector has a resistor that’s built-in, so if you plug it in fairly slowly it will prevent the massive inrush of current that creates the spark.
Here are some relevant threads
Put a fuse between your charge port positive and your battery positive. In case anything happens such as a short on the charge port your battery will be protected as it will just blow the fuse.