I too would be concerned, not so much with softness, but adhesion and clarity. Water based products have to have a catalyst to cure / harden.
Some use extremely toxic substances like aziridine to accomplish this. I would like to know what the Rust oleum products use. There could even be a possible issue with incompatibility? Best as you say to test on something else first then even try a scrap panel made from the same plastics.
Urethane and other poly type based products are proven and fairly solvent resistant which are very important in case of a spill when refueling or a splash that always seem to happen. These can also be applied over water based base coats.