Hello, I just started to play Fallout 3 and I also have bought the DLC packs. I was wondering if I should wait until approach the end of the original campaign before installing the other DLC, because I don't know.. maybe the level cap of 30 would not work on the original world that has been designed to scale for levels 1-20? Are there any reasons to wait or would you suggest me to enable all of them now?
Thanks!

you should of bought the game of the year edition..it comes with all the DLC's.. but nevermind. You will need to install broken steel before the end of the default campaign otherwise it rolls to the credits and ends the game..

You should start to install dlcs after you reach level 7-10...And as Dhegonus said you must activate B.S.
before finishing "Take it back" quest

If I install Broken Steel now there are high chances that I will be level 30 way before reaching the end of the first campaign. Would the original content also scale to 30 or will it become too easy?

you shouldn't reach level 30 if you just purely do the default campaign..you barely reach lvl 20 by the end of it.

Use some slower levelling mod

It is best to install them. The order is to install Fallout3, immediately patch to 1.7 and then install the DLCs.

Check out the Refurbish DLC mod on how to "disable" the message boxes you get about the DLCs.

There are slower leveling mods that help, as well as a mod that re-balances the Operation Alaska equipment. Not to mention mods that add to realism and difficulty. So it isn't really a problem to make Fallout3 more challenging and more even-paced level-wise -- the trick is finding the ones that suit you and your style of play.