garageband and idvd is after el capitan clean install missing

If the installer let's you install, install both applications. Then update them from the Apple Support Downloads page (Apple - Support - Downloads) to the most recent versions.

You need GarageBand 6.0.5 (GarageBand 6.0.5) and iDVD 7.1.2. (Download iDVD 7.1.2) Both will run on El Capitan, but are no longer officially supported.

If the installer does not let you install them, restore the applications from your backup. But you will have to restore the support files from the Library as well:

• /Library/Application Support/GarageBand/
• /Library/Audio/Apple Loops/
• I don't remember what iDVD might need as support files.

It is risky to make a clean install if you cannot be sure, if your installers will be compatible with the system you are upgrading to. A clean install is usually only necessary, if you installed software that corrupted your system.

I could install both applications from the application DVD and after that the both updates without any problems.

That's great, you're welcome 🙂

Generally I prefer clean installations of OS Changes.

Just to explain what I meant by my remark about clean installing:

My experience is, that the system runs quicker and solid and all applications work fine.

Right, it used to be this way with the older versions of MacOS X and used to be good practice to clean install. The MacOS X newer versions are are much more robust and will quarantine incompatible system extensions, fonts, and similar. Conflicting items will be collected in a special directory, where you can see exactly what cannot be longer used in the new system version. This way you will know, which applications you need to update to be able to use them and not be tempted to install incompatible software.

If you make a clean install, you will get a clean system, where many dynamic libraries from the older system versions are missing and there is usually no easy way to restore them. But older versions of your software may need them. For example, if you previously used the Jam Packs for GarageBand, there is no problem to leave them installed, but after a clean install some of the installers will not run, because they contain PowerPC code. Some applications, like Finale need older Java versions, that have to be added again, older versions of iLife applications need iLife Support or older QuickTime codecs, that will be thrown away by the clean install, and you may only notice later that themes for printing, fonts, video codecs, etc are missing. And because of the absence of the quarantined items, you have no guidance what is save to install. The worst thing that could happen after a clean install is that you run an installer for an older application after a clean install, and the application will now install an incompatible system extension, exactly what you have been trying to avoid by clean installing. . An infamous example of this problem is Apple's own Final Cut Express 4. If you reinstall it after a clean install, it will corrupt your pristine system and cause other applications to crash. (See: https://support.apple.com/en-us/HT203405) But if you do not clean install, but install the upgrade on top of your current system, the upgrade will remove the incompatible extension and all is well. And Final Cut will continue to work.

The one thing you have to be very careful with when upgrading are are audio drivers. Old drivers for audio interfaces, audio units, VST plug-ins, etc. can destabilize the system. Because they control the hardware they usually install system extensions and can cause crashes or slow the system down. If you reinstalled any of these items check the manufacturer's website for updates and compatibility. See for example: GarageBand for Mac (v10.0): Unexpected behavior with Audio Units plug-ins

Another question is, how can I check if the installer of an application is compatible with the OS?

First of all, check if the installer will still run on the current system. If it no longer runs on the current system, it will not run on the new system either. Check the box with the installer media for system requirements, and then check the manufacturer's website for compatibility.