For what i ve seen you can use iron or other material attract by magnet, it work, but will be better with another neodym magnet, i made some tests. Regarding the styrene fact, i think it could also be a matter of density. except jfoam who use fitness mat 35kg/m3 people usually work with eva at least 45kg/m3 when it's not more than 100kg/m3, lots of people advise me to use 120kg/m3 so it probably make the attraction weaker while styrene is more like 35kg/m3, i suppose it could make a difference.
By the way, even if you put more than one magnet i still recommand to buy some more powerfull magnets than you plan, just in case, i took some 6kg for larger pieces and for pieces who could maybe flex or bend. It's strange people speak about size of magnet but not power, because it s what is matter in fact, you could have little size with more strengh than a bigger one and vice et versa
also the distance between the two magnet is the most important weakener fact when you use magnet, when i first try the 1.9 i use it on a worbla piece 3 mm including 2mm foam and one side worbla, it s stick really well, i could move one magnet on one side and see the other one follow on the other side, with foam and 1 cm between the two magnet it barely stick so 7 mm made a whole difference and like collinE83 said if you use a thin piece of foam for the purpose of not having a too big distance between the two magnet it could make your foam flex, rip, bad stuff, with styren it stay neat with more attractiv power, i ll use that when i ll build my stuff, good advise