My family and I will be arriving back to the USA on CX888 arriving into JFK at 0600 on 29 Nov. Our connecting flight on DL to Florida leaves at 0800. Will this leave us enough time to clear customs and immigration?

Doable with no checked luggage but tight.
Will not do it with checked luggage.
Don't forget you have to change terminal at JFK.
Delta Terminal at JFK is

I have done something similar before, but instead of JFK I did HKG-JFK/LGA-JAX etc. instead of doing a lot of walking + waiting/taking the train within JFK...I'd rather just hop on a cab exiting the JFK terminal and in ~15 mins arrive in LGA (assuming good traffic of course). I have done LGA-JFK ~9-10 am / 9-10 pm and JFK-LGA ~1-2 pm...usually not bad.

To me, two hours seems too tight. Remember, you will have to clear immigration, claim bags (if any), clear customs, get on the AirTrain, check in at the Delta terminal and reclear security. You may want to play it safe and book a later flight on Delta now. Flights to Florida (esp on weekends, although less so to JAX) in the winer from NYC are notoriously full. If it were me, I'd rather know that I have an extra couple of hours to connect and plan accordingly than risk a misconnect and have to stand by all day. All that being said, I'm sure there are passengers that successfully make similar connections at JFK every day.

Honestly, as a New Yorker, there is no way that I would do this. Like you say, the trip can be 15 minutes. Its also taken me an hour to get from JFK to LGA. It can be a real mess. Plus, in my experience, the delays at LGA are far worse than at JFK. I understand this to be because of LGA's runway configuration. YMMV, of course.

In short, I would said 3 hrs min and 4 hrs to be safe.

Does it help if we're flying J? Not sure if JFK has preferred immigration lines. Also, shouldn't things be pretty dead at 6am? This is also 29 Nov - the Sunday after the Thanksgiving holiday. Probably a huge travel day, but I'm guessing 6am immigration queues will be short...Plus Delta has changed the schedule again - now 8:30 departure to JAX. 2.5 hours now. Next available flight to JAX on DL isn't until 13:40.

Part of the challenge will be escaping the CX terminal, trudging up to the SkyTrain (or is it AirTrain at JFK?), waiting for the right train, and heading to the DL terminal. All three variables can be very short, or quite long. After flying CX to JFK last month we sailed through immigration but got held up for 15 mins in the queue to hand in our customs form. Then we had to wait 8 mins for the right train. The train goes only in 1 direction so if your destination terminal is 'behind' you in the route then that adds more minutes to the journey. All told, these three variables if all working against you can create a significant delay to your journey. I advise checking the train route map at the JFK website. Maybe they give train times as well, because the frequency at 6:30am may be different than at 7:30am.

I missed my flight with a 2 hour layover at LAX.
Aiport employees excuse. too many flights landed at once.
But I'm urgent, Too bad they said. wait in line.
No sense to open another line for urgent passengers and have someone check tickets as a confirmation. they just stood around, had to ask my way to the front, some said no, so I was stuck halfway to the front and missed my flight by 10 mins.

Were you arriving from an international flight? LAX is really bad for international arrivals! (It is also bad for departures too).

Yeah i was on an international flight

If you have a choice, try SFO instead of LAX. It is MUCH better.

Does this flight via YVR not pre-clear Immigration and Customs in Canada?

No it doesn't.

The DL terminal at JFK is horrible when I've been there. Ridiculously long security lines, check-in lines (even for kiosks - OLCI before you leave YVR!) are horrible, etc. i usually have flights out of terminal 7 in JFK (used by CX, UA, BA, AC and some others) and whenever i have DL flights, I'm amazed anyone makes their flights at that terminal.