Pepe,
Mavericks cannot install FCP7 from disks because the installer was PowerPC based, hence the error as Mavericks will not run any PowerPC installer. This is not a Rosetta issue, just that Apple never provided a updated installer for FCP7.
Assuming you still have your old computer, you can install and run FCP7 on Mavericks (no Rosetta needed) by using Migration Assistant (MA) but unfortunately, MA is an all or nothing program. If you are transferring all the old programs, then MA is the way to go. You can also delete the programs you don't want before or after MA transfer.
FCP7 will run on Mavericks, it is just getting it installed that is the bear.
You also can't simply drag the application over by USB unless you know where all the supporting files are located on your old computer, copy them and then install in the proper places on the new computer. Unfortunately, Apple has changed where each file goes, which is exactly what an installer program does, puts all the files in the appropiate places. Lots of hidden files.
While it is possible to use terminal to gather files from the old computer, and then to place them in the appropiate folders on the new computer, it is not something easily done.
If you don't want all the old programs but just a few, one way to cheat, is to clone your old hard drive onto an external drive, remove all programs you don't want to transfer, and then run MA. Big Pain.
Having said all this, I am a big fan of the new FCPX 10.1.1. Way faster, better management, no out of sync audio, much better multicam, etc…
You can download a free trial to play with it.
Check out FCP.co for some great articles and forums.
Hope this helps, Greg