Use whatever your horse most enjoys for rewards. Scratches are great cause you always have them "at hand"
I'm also the dissenter here as there is nothing wrong with using a food reward as long as you teach them that grabbing, biting, being pushy is not allowed. As someone who uses positive reinforcement it is all in how you do it. All of my horses are first taught, behind a fence or stall wall, that the way to get a reward is to not grab it. Once they turn their head away they get the reward. Reinforce the behavior you want. If they do start getting amped up then walking away is the best thing as that removes any reward by you being there to play. If any horse is still grabby with treats during training you can always have a pan on the ground and drop things in there instead of feeding by hand.
Have fun with your new "kids"!