Keep in mind that the layer of primer and paint already on your model do protect the plastic itself to some degree.

Enamel thinner/white spirit/mineral spirit, even from model specific manufacturers, are known to cause brittleness and cracking in polystyrene. But those products can be used prlerfectly safely over a lacquer or acrylic base.

I strongly doubt that the rustoleum is going to harm your model. It might harm your paint to some degree if applied too thick and too wet, but judicious application makes virtually any combination of layers of paint types and solvents possible, if not ideal. There is, IMO, too much fear-mongerong around "non-modeling" products floating around these days. I successfully use plenty of things designed and sold for other applications in my work, often because I have found them to be superior. For example: Testor's brand enamel thinner is SPICY. Too spicy in my opinion for delicate weathering processes and more likely to damage previous paintwork than I would prefer. Instead, I buy large jugs of "Oderless Mineral Spirit" from an art supply store which are simultaneously cheaper, gentler, and less stinky then the "designed for plastic models" product from Testor's. That's certainly not always the case and often hardware store spray cans are of lower quality than their modeling counterparts, but they're not necessarily something you should gonout of your way to avoid at all costs.

Not here to say that you shouldn't be cautious, but I would encourage you not to be afraid of generic or non-modeling products in your work. Play around with different things until you find the combos that work for you and for your style and flow.