Sure tank drilling is fairly common expecially for the larger tanks.
Its fairly unrisky as long as the person drilling knows what they are doing and has the right equipment. This is defantly not something to be done by yourself. If only the bottom is tempered then just take the bottom out of the list of possible drill points.
Research in your area if you have any LFS
's or glass shops that will do glass drilling. Some LFS
's will drill tanks but I find they can be kind of costly. A local glass shop could drill it and they are often fairly inexpensive at only a few $ per hole drilled.
You need to figure out the size and placement of the holes you wnt drilled. Most will use a diamond tipped holesaw designed for glass. The only big danger comes if you dont keep the drill bit or the glass around the cutting cool.
To size your holes you first need to decide what size of plumbing you want to use and then size out the approprate bulkhead for that size plumbing. For example I had a hold drilled for a 1" pvc
plumbing but the bulkhead for that size hold needed to be 1 3/4" in size.