I just tried to follow some directions I found with google to limit which contacts showed up on my phone. It involved putting them in another group then removing that group from My Contacts. Well, I couldn't get it to work, lol. It's a shame too, I could use that feature myself.
You do not have to have your phone sync contacts with gmail at all. If you go to settings and accounts and sync and press on Google, then press on where it shows your account. You can select not to sync various google products. You can just have what contacts you want on your phone in it's contact app, called people. It should allow you to choose if you want to just put it on your phone.
I am not sure how to get the contacts already on there off though. I don't know if they will go away when you choose not to sync them or if you will have to use menu/remove to take the whole account off the phone to get them off and start over.
You could also deselect gmail from syncing and I think you could then just sync from within gmail itself. I don't believe you would ever get your gmail till you did though. It will wait till you tell it you want it to check. That might not be such a good thing?
Hope this helps. This is a learning thing for me, right here too, so I am open for suggestions or corrections.
Oh, and to delete a bunch of email, I would use a browser, either on the phone and log into gmail or from a pc.