I can't say what weight your tires will be able support but my experience has been that most RC vehicle tires are rather flimsy. Typically these tires support all the load with the rubber of the tread, there is no air pressure inside the tires.
I wouldn't count on the tires carrying much more weight than the cars they were designed for, however Nitro or higher end car tires might be designed for a bit more load than your toy store RC car.
I have also seen foam inserts sold to stiffen these tires, im sure you could make your own inserts as well.