6000/600 Equalizer is definitely underrated for that trailer. It's a recipe for porpoising. 10000/1000 will be perfect.
I tow our 5200 lbs (loaded) trailer with the 10k Equalizer, and it's fantastic. When in doubt, round up.
I suspect you could find a buyer for your used 6k Equalizer.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller