The answers to your questions were a little vague , so I'll try and add a little more detail .
Ground frequencies are both 75 and 27 ( and 50 if you has a Ham'ers license to operate )
All of the spectrums are acceptable , as long as they are either 27 or 75 Mhz freq's ( meaning - AM - FM - PCM )
Also , cars and boats share the same freqs .. called ground freqs and air is seperated ( for quite obvious reasons , although it would have been nice to seperate all three with each having their own freq ... )
Next , if you have a park flyer and radio gear that gives you 2500 feet , that is 1000 feet more than manufacturers specs ( but , this depends a LOT on what kind , and how much interefernce there is around you , along with typical ground noise )
A radio is maxxed out at 750 mw of power no matter which radio , what freq , or channel you run .
If you are going to a park where others run , it would be a VERY good idea to first find out what channels and what freqs they usually run on so that you do not match up the same .
Although the FCC is NOT out and about , at every local pond , air field and race track ( for cars and trucks ) , they would be the ones to issue you a fine , but first , and foremost , it is more of a common courtesy thing about running the correct freq ( and the law also ... )
Since you run park flyers , you should understand this .... Think if some mo-jo went and was running his Traxxas on 72 Mhz and then , even the same channel , and was within an area that would affect the way your plane handled ... hmmmm .. bad news , huh ?
So ... be considerate of your fellow RC'ers and run the right freq(s) and also check with the others that are near you for possibility of same channel .