I wouldn't say that they're "perfectly compatible." As far as I know, "grass frogs" in the US can be applied to multiple species of frogs. Knowing exactly what species would help make a better determination here. I personally would not recommend housing them together. They may be found in the same areas outside your house, but I highly doubt they're found inches from each other for long periods of time. You will be stressing them, which will cause them to release conflicting toxins that can make the other one ill if the stress alone doesn't do it. Not to mention, American toads can grow pretty quickly, and depending what species of frog you have, could end up outgrowing them and eating them. If you're talking about Pseudacris ocularis, (sometimes called little grass frog) that will surely happen. Tank size and exactly how it's set up can make a difference too. If you're set on keeping the found toad, I would get it a ten gallon tank and house it on its own.

In my yard, I can find grey tree frogs, american/fowler toads, leopard frogs, and green frogs (R. Clamitans,) spring peepers, and bullfrogs feeding/breeding/soaking in the same areas, but that doesn't mean they can be housed together safely.