I have a few (~5) applications where the version in the debian stable repos is to old for me. Examples would be podman and restic. I found out that the version for testing are recent enough. Is it possible to install specific apps from testing while the default is stable?
Yes, this is possible if you configure preferences (see man apt_preferences). However if you install numerous packages from testing, this can lead to various bugs. I used to do such installation and concluded that it is better to avoid this.
With that said you could also check if the packages you require are available in the backports repository. It doesn't look like Restic or Podman have packages available but its possible they will during Trixie's lifetime.
That was many years ago, so I don't remember all of them. E.g. Chromium was unable to display pages (was it installed from testing? or it was from stable but some of its dependencies from testing? I forgot).
For some apps that are not system relevant such as restic I have found a solution in form of distrobox. But that does not work for programs like podman.