Bonjour, Stef! Comment ca va? That's about the limit of my French these days...
Firstly, welcome aboard!
Ninjutsu is an interesting art, which is taught in different ways in different schools... so just how applicable it will be for security uses will vary depending on which school you are learning at.
The major groups (authentic) are Bujinkan, Genbukan, and the Jinenkan. All of these schools trace their teachings back to Takamatsu Toshitsugu, a man claimed to be Japan's last ninja. Takamatsu-sensei was the lineal head (Soke) of a large number of martial traditions, including samurai arts (jujutsu) and ninja arts (ninjutsu)... although that's a bit more complicated than that as well....
To take the groups in reverse order, both the Jinenkan and Genbukan focus more on traditional training and methods... which don't necessarily lend themselves immediately to modern contexts and usage. The Bujinkan, on the other hand (the largest, and most widespread of the organisations) is much looser in it's hierarchy and structure... exactly what you learn, and focus on, will be in large part down to your teacher. I've seen Bujinkan dojo that are highly traditional, and ones that are almost nothing but "modern" expressions... and anywhere in between.
But the question is how applicable to security work is it? Well... that really is down to the instructor. There are a large number of Bujinkan instructors (as in any art) who have experience in security work... and, being a Japanese art, there is a large emphasis on stand-up grappling (locks, throws, chokes, pins etc), which can transfer quite well. At the end of the day, though, any art will give you more of a sense of your own bodies capabilities and limitations... although, for security, I'd aim towards a grappling art, such as judo, BJJ, or a (legitimate) Ninjutsu school. I do emphasise "legitimate", though, as there are a large number of schools who have no basis in any historical connection to Japan let alone any genuine Japanese arts.
If you do have any further questions on these topics, please post them. This area doesn't get a lot of traffic these days, and the answers commonly come from those with no experience of our arts... so I am happy to answer anything I can.
