>How do you install a side rail without fucking things up?
If you know how to correctly drill and tap a hole its simple provided you have the tools to correctly do it, i.e a drill, tap and vice that can hold your shit. If you aren't competent enough for that/don't have the tools, the gunsmith will probably do it for 50 or less.
>What kind of optic is good for a shotgun?
A red dot, not even lying. Its like you're cheating at skeet and trap with one.