I don't have a v6 mustang but know a bit about nitrous.
First off you need to decide if you want a wet or dry kit. With a dry kit you run the risk of running lean from not having enoug fuel.With a wet kit you run the risk of puddling the intake if you spray too low in thr rpms. Personally I like wet kits only because you get
fuel and nitrous injected to make sure you get enough fuel. But if you are going with a small shot I suggest a dry kit. It's all up to you though.
It's all on you and your supporting mods.
I suggest you start out with a 50 shot and then go up with a 75 shot.I don't think you should go 100 shot because you may run too lean.
The things you will need to ensure safety are
colder spark plugs,bigger fuel pump,decrease timing and don't forget 93 octane gas is a must! It's best to put it on a dyno to see if you are running lean or not too.A tune is also good.
I also recomend a WOT switch and a window switch for added safety.
If you do it right nitrous can be a safe power adder.