For the vehicles I find it easiest to just put them in the latest patchday replacing the police police2 and police3 so that the AI will use them, for the peds I'm having the same issue as you, and I can't find anything on it yet.
For the peds using the default lspdfr backup menu fixes this issue usually, and for me It's ultimate backup that is spawning default peds. But It would be nice to be able to use ultimate backup without the default peds.