The are with different scaled, and in case of station buildings, they may expect the track at a different position. Also a lot of these buildigns may overwrite similar buildings with the same name (most likely for townhalls and city buildings)
In order to do so, I advise to create a folder "addons" in you simutrans directory (where "save" with the savegames is) and then inside a fodler "pak128" where you copy the buildings you want. So you can still join online games with the vanilla pak128 and use your pak128 with the additions.