Hello,

I've got just a quick question:

Is ist possible to use other Maps with GTA IV and LCPD:FR without any serious bugs?

Well there are a few out there, but you have to think like this ( I don't know how powerful your computer is ) Every map you load is more textures, which gives chance on city bug :)

No.

LCPDFR is not compatible with any modification which makes changes to the core map.

The reason it isn't compatible is that LCPDFR's callouts have preset locations, based in Liberty City. They are not set up to work in modded maps. The core functions besides callouts should work (arresting, traffic stops, pursuits), but not callouts. Also, if the map replaces Liberty City, then the police stations will not work either, but if it adds on to LC, then the original LC PD's will work.