I would recommend 500 minimum.
Should be on top of the starter - follow the + battery cable
disconnect battery
check your regulator. it maybe over charging your battery
Yes, if it fits the holder.
Under your battery tray
It is hooked in with the driver side battery to provide more cranking power.
My 2001 Chevy Blazer's security stays on. Does that effect the cranking up of the vehicle?
Yes, it should fit perfectly.
Group 75.
Discconect the negative cable from the battery for a few minutes and that should do the trick
Disconnect the negative battery cable from the battery.
Your 1999 Chevy blazer doesn't have a timing belt.