I disagree about walking where they are lots of dogs. In my experience the "popular" dog walking areas are filled with poorly socialised "but they only want to play dogs" as they charge up to you and your dog.
I would initially walk at quiet times and quiet places. I would join a good training school and meet people and dogs and then join up with the suitable ones that you and your dog are comfortable with. You can then walk together in the knowledge that the dogs will get on. You will gain confidence.
It is not too early to contact trainers before you get your dog, go and watch some of their classes, good trainers like to see clients before they get their dogs to help them with the issues you are thinking about.
I do not think it is irrational at all to not trust dog owners - I don't either! I do not let my interact with dogs unless I know the dog and the owner. Dogs do not need other dog interaction - you are more than enough for them.
Interestingly a very talented dog trainer Steve Mann was recently talking about small dogs. He said (and I agree) that if a dog was making his small dog feel uncomfortable he would pick it up and move away. I totally agree with this. I would make sure my dog was safe and reassured and then work on the issue when the dog was under threshold. There is an old fashioned view that this will make your dog more anxious - it will not.
A lot of people think small dogs should be left on ground level to sort things out - this is an outdated cruel idea.
I agree about body language but practical things you can do are:-
Find a quiet dog walking area
Carry treats and if a dog does charge up to yours throw the treats out to the charging dog pick up your dog and move away
Practice calling out "Can you call your dog please" and do not hesitate to shout this to the owners. I doubt they will be able to recall their dog but they will try and run after it and give you time to grab your dog and move away.
There are some things we can teach your dog too:-
Teach your dog to stand behind you, or in the middle of your legs, it is hard for an approaching dog to get to your dog in this position (My dogs are taught to stand on my toes if they are anxious)
Do pay attention to your gut reaction - if you are not happy with an approaching dog then change direction .
Most of all though enjoy your dog, find a good trainer who will give you confidence to keep your dog safe, it is not an irrational fear at all.