you can use any slot, if you want to be able to select the vehicles in the police station without spawn it, and also see it on the highway you have to go to Grand Theft Auto V\lspdfr\data and edit the file agency.xml, make a backup of the file, take time to read it, look for this lines
<Name>San Andreas Highway Patrol</Name> replace the cars or add the cars you want to see on the highway