Add on vehicle pack explanation request

Hi, getting back into LSPDFR and trying to get a more fully immersive experience than in the past, so I'm downloading some more in depth mods including EUPFR Basic Configurations which suggests you download some add on packs for vehicles. I would love to do this, but I'm not so clear as to how these 'add on + replace packs' work.

The instructions are very clear, but I don't get what the final result should be. My understanding is that once you download the LSPD pack for example by 11john11, and properly install it, all the LSPD vehicles (police 1, 2, 3, t, etc.) will be replaced plus a few more will be added, then in accordance with EUPFR configurations and sometimes ultimate backup, these cars can now be selected through the police garage, they will spawn at random like other police cars do, and be able to come as backup if requested? Does this also include say, the BCFD or FIB pack? If I request specifically FIB police backup from ultimate backup, the FIB police add on (not replace) will spawn and work? And the specific BCFD pack will come if I request BCFD, and I will see those at random in Blaine County, but not in LS?

Very new to this type of modding so forgive me if it seems like I answered my own question, but I'm just simply not sure. To me, as someone who hasn't modded LSPDFR / GTA too much before, this sounds awesome and honestly, too good to be true.

Also as a side note: if what I explained in the main paragraph is actaully the case with these add on packs, should I just go ahead and install EUPFR Ultimate Edition instead for even more immersion and variety?