Can you request VFR flight following for your local non towered practice area? Can you have flight following without filing a flight plan? Can you also receive FF when flying low (800 AGL) locally for separation from pipeline patrols and ag spray planes?
(I've assumed you're asking about the US because you used the term "flight following". If you ask about regulations or procedures, please always tell us which country it is and use a country-specific tag if appropriate.)
Can you request VFR flight following for your local non towered practice area?
Yes. I did a tower tour at Huntsville, AL (KHSV) once, and the controllers asked us to request it in the local practice areas. First and most importantly, it helps improve safety and traffic management. But as a secondary point, it counted as an "ATC contact" or whatever they called it, and that helps to justify the existence of the ATC services in that area. That's a good thing for both controllers and pilots.
In general, ATC strongly prefers to be in contact with aircraft in or around their airspace, and I've heard that in multiple presentations from them. If they're really too busy to provide the service then you won't get it, but it never hurts to ask.
Can you have flight following without filing a flight plan?
Yes. They're complementary but not the same thing and they're managed by different groups: Flight Service manages VFR flight plans and ATC manages flight following. In fact, ATC doesn't even have access to VFR flight plans.
Can you also receive FF when flying low (800 AGL) locally for separation from pipeline patrols and ag spray planes?
Maybe. ATC needs to see you on radar to provide flight following and if you're too low you may literally be below the radar.
For the practice area, a better term would be "Request traffic advisories." Flight following implies receiving radar handoffs to subsequent controllers along your route of flight. The former involves only radar identification locally. The latter requires an entry into the National Airspace System to process the handoffs from facility to facility.
A VFR flight plan is purely used for search and rescue and is completely separate from flight following.