When I started out HST I did the 15 rep squats first and SLDL 2nd. - I never did more than 2 sets. Also, the first set was a warmup set. This way I only had one workset.
It helped, because I was fresh.
I don't eat, even today before workouts. I take a couple fish oil caps and 48G protein shake with water only. If I ate before workouts I'd be puking on the heavy exercises.
Now I can work in squats and sldl just about anywhere in my workouts, but when I started I had to be careful. I don't know what could have happened, but I remember one day I just sat down and then had to lay down on the floor. It was not an alarm kinda thing for anyone. I just walked over to the side of the room and sat down. That was after about 3 tough exercises, squats and SLDL and some incline db presses.
Honestly, I just think my body wasn't used to that much exertion. Remember, you aren't going to make a huge headway in a day. Give your body time to prepare for the exertion. Listen to your body, and you can do that by taking more recovery time between reps and exercises.
Currently, I do 1 minute max and usually less recovery time between sets and 3 minutes max time between exercises. I do this to give me somewhat of an anaerobic training to boost my metabolism.
I'm in the tens cycle now and I completed my last workout of 10 compound exercises in just over 45 minutes. I consider my training a serious matter, and my workouts are not social events for me. I am always cordial, glad to help... but if anyone wants to train with me they have to keep up the pace or I just move on without them.