I think you are taking a risk by loading the horses in the front. Yes, the weight should be balanced so that 10% is on the tongue in a tagalong trailer, and 15-25% is on the tongue in a gooseneck, but the risk of not being able to unload is a significant one. Does your trailer have a front escape door that is actually useable? (Many escape doors are not usable for getting horses out.) If so, then you are fine. If the only exit is off the back of the trailer, I'd switch things around. Put the horses in the back. If you have an accident that is so severe that it penitrates and destroys the rear portion, you have significantly more important things to worry about. For example, the shock of such a collision will destroy the tow vehicle and will probably severely injure or kill the horses anyways. Minor wrecks aren't that big of a deal and will keep the horses safe regardless of where they are, unless they happen to be leaning on the point of impact. Think of it this way... if you have a cart in the trailer, and you get in an accident that is severe enough to damage the rear gate to the point where its barely openable, how are you going to get in to treat the horses' injuries? Or get them off? They are locked in the front of the trailer, unable to be unloaded or even gotten to. You generally can't open a center divider with a cart loaded in the back. And you can't get the cart off even if you can get the door open enough to climb in. If the horses were in the back you could unload them through the smallest opening, or get on board to do treatments and emergency care. You can't do that if you can't get to them.
It is also not unheard of for fires to start onboard trailers. If the horses are in the inner compartment you will have to unload the cart first in any emergency to get to the animals. If you pause on the side of the road for whatever reason, you will need to unload the cart on the side of the highway before you can get to the horses.
In my professional opinion the draft driver is correct, load the horses in a compartment that is easily accessable. If you have a cart and heavy horses you will have to purchase a special trailer which takes into account the biased weight (axles will be differently arranged). In your case, 600lbs is NOT going to make a BIT of difference in your load. Yes, it is PROPER to load the heaviest weight forward, but loading 600lbs more in the rear than the front is NOTHING. Especially if you have a gooseneck, and DOUBLE especially if you have a tackroom.
I routinely load more weight on the back of my gooseneck trailer than the front. I do this for balance, to get the tongue weight down to 20%. That is MORE than enough to avoid sway and to keep everything safe. The rear axles in a gooseneck are generally farther back than a tagalong trailer, and can easily support the weight without "see-sawing" the trailer and lifting the front end. If you have a gooseneck trailer I'd immediately swap and put the horses in the back without a second thought. If its a tagalong, you'll have to do some engineering thinking and possibly some weighing at a public scale to see how your trailer balances. But again, we're talking three minis, not a team of drafts. If your tongue weight changes by 50lbs I'd be surprised.
As far as bounce, that depends on what kind of axles you have and what kind of truck you are towing with. My trailer's 7k axles are SOFTER than my truck's stock suspension. So the gentler ride is over the trailer axles. When I added air suspension to my truck that kinda cancelled out the effect, so both are very soft. A dually truck will have a far rougher ride in the front than it will on the trailer axles, if they are new axles with soft suspension.
