@Abdul Rasheed, at the end of the day, successful athletes - one could expand this beyond athletics - are people who know themselves: their strengths and how and when to use or conserve them, and their weaknesses and how to strengthen them.
Will strength training lead to muscles being too tight? Sometimes, yes, sometimes, no, and often it depends on the person - the same exercise may create a need for stretching in one person but not another.
I recall a conversation with Pavel a number of years ago when, for reasons I don't remember, we were discussing how to sum up StrongFirst principles in a single phrase or sentence. I believe is was Pavel who suggested "complete mastery of muscle tension."
Pavel has observed that most sedentary Westerners live in the middle of a continuum with total muscle tension on one side and total muscle relaxation on the other. The problem is that, while they are "balanced," they are only balanced because their lack of skill prevents them from away of the middle. A life well-lived should include the ability to move to both ends of the muscle tension continuum, or at least not to be stuck in the middle.
In our Flexible Steel workshops, we talk about using your strength to improve your flexibility. Pavel's books, which are the basis for and background against which Flexible Steel was created, discuss this as well. Be Strong _First_, and with your strength, strive to improve your flexibility. A weak muscle doesn't want to relax but a strong one is more willing. One of the "light bulb" moments in my training was realizing my splits improved when I started training pistols.
In my mind, stretching does not counterbalance strength; stretching is a demonstration of one aspect of the mastery of muscle tension; the primary aspect of the mastery of muscle tension is strength. Only a fool wants to be flexible and not strong - studies have shown these people are at higher risk than those who are less flexible. Strive to be able to demonstrate both tension and relaxation throughout a complete range of motion. Maintain the range of motion through which you are able to demonstrate flexibility in at least the same neighborhood as that in which you can demonstrate strength.
To address another of your questions, the difference between mobility work and stretching is that in the former, we are using a range of motion we already have, making sure we don't lose it, making sure our joints are well lubricated, and making sure our movement patterns are sound; in the latter, we are attempting to increase our range of motion.
If you are having trouble understanding the meaning of the word "conditioning," you are not alone. I wouldn't be too concerned. One "conditions" one's self to an activity by performing it, and you will become conditioned to your training. "Endurance" is a word often confused with "conditioning" - far be it from me to try to unravel that one, except to say that I wish to be conditioned to the activities of my choosing; becoming so conditioned happens through my body's adaptations as I perform those activities repeatedly over time. I don't want to engage in activities that I have to "endure" except as my life might require such endurance. If you ask me, sitting on an airplane is an endurance activity.
-S-