Did you have to make a topic when you asked the same question in the Additional Units topic? I don't think they are supposed to be able to return to the fire station. Thay are bigger than the normal ones and this could have caused problems.
You don't edit the config file. You have to edit fp_params_endless.xml which is in the specs folder. You then change the initial event wait value and you just have to edit it and try it until you find something you like. BTW to the forums.