You will need an EC2 cable to charge the ST16 battery. Also note, you need to reconnect to your wifi with ST16 after changing batteries in order to update the time and date.
The ST-16 battery is a single cell 8700mAh... so 8.7 Amps would equal 1C... I would say if you charge at 6A, you will be fine... since it is a single cell you do not need to concern yourself with any balancing.
That would work, but it is generally believed that the OEM batteries were under rated on the packaging to be imported as cheaply as possible... 5 amps would be playing it safe. It definitely will pay off in the long run, to never try and push the envelope with these batteries... not in use and not in maintenance. Those seeking that last tenth of a volt before landing, are shortening their life. Those not bothering to discharge to storage voltages within 2 days are shortening their life.
I replaced 3 of them before I got it in my head...