I currently have a 2006 f-250 diesel ext cab long bed, I absolutely love it and can't afford a second...
i need to pull a 4-horse goosneck trailer GVWR is about 6900lbs and length of trailers range from 28-32 feet.
Is it possible (and safe) for me to pull this with my truck or do I definitely need a dually?

If def dually what trailer length would my truck be safe hauling?

 

added weight of horses would be up to 5500lbs (so about 12,000 lbs total MAX)

 

Not sure what your pin weight is but I pull a 31' 5vr that comes in at about 11980#'s loaded for a week. My pin weight is in the 2300# range and I am safe on all axle rating numbers, and total combined gross weights. My 05 is rated at 10,000 GVWR, but I did add a set of super springs.
I have pulled and stopped this set up thru mountains with no problems so you should be ok.

 

you should be more then fine

 

I towed similar trailers with my old supercab short bed V-10 truck without issues

you should be fine