If the trailer is balanced correctly and the tongue weight is right, it shouldn't fishtail. My hitch on my old suburban was 2" too high and my trailer (dual axle) swayed a bit over 70. Dropped it 2" and no sway. I've towed an '89 Comp from Missouri to NH and it didn't sway either (single axle).
There is no reason why the surge brakes can't work, if everything is moving correctly. I have seen many, many functioning surge brakes on boat trailers, mine included. Backing up, I always use the lockout. Most of the time using the truck brakes it won't matter, especially towing with a 1-ton truck, until you have some emergency braking situation and then you'll feel stupid for not getting the extra braking from the trailer.
1989 Bravura - repowered with 2001 PCM 351 GT-40 - 1.23:1 PCM Trans.
1989 comp - sold
1985 comp - sold