do the couplers pivot with the trucks? If they do, they will be able to go around much sharper radii than those with body-mounted couplers. I have seen IHC full-length cars go around 18", and they work, but look weird. And, you need greater clearances around curves because the ends overhang. Diaphragms petween the cars simply won't work.
Around 24", the cars still look weird going around a curve, but not as weird as on 18". They will probably operate fine around 24", It's just that they may not look right. Diaphragms again may not work well.
I've always figured I would need 30" minimum radius if I wanted to run full-length streamlined passenger cars and have them look okay. If you just want yours to run, they may work on 18" with a little modification.
Kevin