I don't think you would have any toxicity concerns, but I am not an expert on that. What I am thinking is that you have a 24 gallon container that you will reduce to 16 gallons if you use styrofoam for 1/3rd the volume. That is roughly a 2 cubic foot container, basically what a commercial earthbox is.

4 plants in that volume of soil is serious over crowding. The planting guide published by earthbox says 2 plants per container of that size, but I only grow one. While your mileage may vary I find the yield per plant drops when more than one is grown in that volume of potting mix. 4 would just be too much crowding of the roots and foliage.

The main objective of using styrofoam partly to reduce the expense of the potting mix and to reduce the weight of the pot if you find you need to move them from time to time. But yes, it would come at a cost to your capacity.

I grow tomatoes in my greenhouse in large rubbermaid tubs. The tubs are very heavy and so I added stryofoam to my containers this yr. It worked great, but you would have to be sure you have enough soil space for sure.