Q:

Must every task in MS project be linked with each other?

The project is composed of three groups of teams, each team will contribute to the project plan. The team has listed their tasks, my question is whether all tasks should have a predecessor and/or successor or it's fine is some tasks can stand alone without linking to other tasks?

A:

Sensei project solutions (I'm not affiliated, I'm just impressed) has a set of quality filters that they suggest you run against every project you document.  One of those filters requires that every task except the first and last have both a predecessor and successor link.  Alas, Sensei doesn't explain their quality standards, they just assert that projects that follow these standards have fewer problems, and I've found that they're usually right - if my project violates these standards, investigation generally convinces me that the standard is right.
What are you trying to do with MS project? 
OK - mea culpa, the most frequent reason to use MS project is because the boss expects it, and the only value of MS project is to produce pretty charts that adorn the wall and convince others that we're project managers. Engineers get technical diagrams, we need something on our wall.  But let's ignore the most common reason, and assume that we're using MS project for analysis, to spot the critical path, and to analyze how to manipulate the project schedule to achieve the most efficient allocation of time and effort.  
If task X is late, or delayed, what happens to all other tasks? Can I do Task y early?   Why document a task that doesn't participate in relationships?  Just put a post it note on the wall.
In my (limited) experience, tasks that have no predecessors, actually depend on the start of project, the approval of the charter, or some other administrative milestone.  If not, then they depend on a resource availability that really converges with an approval milestone.  
Of course it is also possible that you're documenting tasks for resource conflicts. Again, my experience is limited, but that way lies madness. I don't think I've ever seen anyone do real resource management through MS project - I've seen people pour hours and weeks of analysis down that hole, and it seems like the analysis never ends. One of my colleagues was filing over 100 change requests a month for his project schedule.  At that velocity it is pretty certain that the workers and managers are off doing the job while the project manager is filing five change requests a day. 

