If you like animals and think you could handle the care and lifetime (10-15 year committment), you might consider getting a dog (I would say get a rescue dog over a year, no potting training, no teething, and you can usually get a lot of details about the animals personality and they usually do basic training as well). It also sounds like your DS likes animals. Depending on his age and level of responsibility, he may enjoy caring for an animal.
Not that a dog is a security system or even will do anything about an intruder (most won't), but I find that having my dogs around makes me feel more secure because they do react to noises, are there to greet me when I get home, and if someone knocks on my door, my dogs go crazy barking and that does make me feel a sense of security. I also have big dogs, so that probably helps. A Chiuahaha might not do it for you.
Hugs. Living alone can be hard, but you get used to it and you will like the freedom to keep things the way you want.
DS 2008, DD 2010, DS 2013