[*]Can I develop ANY B&W film (not chromogenic C-41) with ANY developer? Meaning can I develop a Tri-X with ID-11 for example?
Yes
[*]If the answer on the above is yes, were do I find the necessary dilution instructions?
On the instructions included with the developer
[*]Do the developers come in different versions according to the film speed, or there Is just one and you play with the volume, dilution etc?
No, but: you may choose different dilutions to manage development time. More dilute solutions take longer, more concentrated solutions develop faster.
[*]Is it safe to assume that using same brand film+developer will produce the best result?
No. "Best" is a purely subjective decision/feeling. Some here use coffee as a base for their developer, some use the generic versions, some off-brands. It doesn't matter as long as you get the result you want.
The advantage for you in using something like ID-11 or D76 or Xtol is that there are a lot of people here that also use them, if you have a question, you can get an answer. Right now I'm experimenting with RolloPyro and the user base is pretty small, tougher for me to get an answer.
[*]Is "stopping" using just water and not a dedicated stopper advisable?
Consistency is the most important thing, using a real stop bath can help with consistency. It does also protect the fixer.
If I were going on vacation and planning to develop film along the way, personally I'd leave the stop bath at home and use water. At home I do use stop bath.
[*]I was told I should choose 1-2 films and developer and stick with them, trying out different variations of exposures/developing, good advice?
Yes, photography is a system made up of many parts. Any change you make, anywhere in the system, has an effect on the end result/the print.
For example this includes lenses. Holga cameras are a lot of fun, but my Holga makes low contrast images so I've learned that I need to add little extra development when I use it, same for my 150SF lens on my Mamiya RB67. Most of the rest of my lenses make more snappy images and don't require extra development.
The problem is that if you are switching films, developers, papers, blah, blah, blah, at the same time you are switching lenses (or whatever other variable), it becomes a real problem figuring out which piece of the puzzle is causing a problem
or a success.
The advice to stick with one film has two benefits/intents:
a) eliminate variables
b) learn what that film is capable of and most films today can do a very good job in most any situation, the differences are not in "quality", only in "character".
[*]Was thinking of starting out with either Tri-X 400 or T-Max 400, any good developers recommended for them?
ID-11 is a great choice, it's good enough that you might never need nor want to change. D-76 is just as good, in fact nearly identical to ID-11 in the way it works. Xtol is good too, Tmax, DD-X, ...
To be blunt here, when starting out, it doesn't matter that much. Yes, developers can make a difference in the print but, to exploit the differences you gotta be pretty good at everything else.
I would suggest that if you do choose two films, they should be significantly different in ISO rating. So choose Tri-X or Tmax 400, not both. IMO it won't matter which of these you actually start with either. Then depending on your style/subject preferences maybe Tmax 100 or FP4+ or Delta 3200 as the second film.
[*]Do you recommend I should go for the 5Ltr developer or the 1Ltr will suffice for some time?
5