You transfer your character to these games or you start with a level 1 character? If you can transfer your character what's the point to play more if you're level 80 and wearing set of deadric armor?

Installing Tribunal/Bloodmoon just adds new locations to the game.

You can start over if you want or you can load one of your previously saved games.

C

I installed tribunal after defeating Dagoth Uhr, and so i brought my pumped up character to Mournhold, nothing wrong with that. The opponents are quite a bit tougher there, so i don't regret it one bit.

you can install Tribunal anytime but due to some quest bug you ought to install Bloodmoon after finishing Tribunal. I reckon this will be fix in a patch.

To clarify just slightly, you don't have to actually finish Tribunal before installing Bloodmoon. They just recommend that you install Bloodmoon after installing Tribunal, or just by itself after Morrowind; the point being, don't install Bloodmoon and THEN Tribunal (due to some sort of software sorting stuff, it just creates problems).

As things are now (no patch yet) you have to finish the main quest in Tribunal before installing Bloodmoon, otherwise it will be broken.

So if you want to finish Tribunal, do it before you install Bloodmoon.

C

Tribunal and Bloodmoon are built to compensate for your strength: either the enemies are leveld on you, or they spawn in bigger groups.
I wouldn't reccomend Bloodmoon to chars under level 30, though. Tribunal can be beaten at lower levels, I heard. But I never checked.

Actually, Bloodmoon can be beaten at lower levels. but Tribunal need a bit higher.