I have yet to acquire an additional G. Rosea to attempt feeding young toads (around 1" to 1.5") to tarantulas. I have fed small toads to my P. Imperator 3 times so far with okay results. To date, I have not fed any amphibious critters to my Ts and I hesitate to do so on a few levels, though I hope to experiment with this in the very near future.
Being Fire Belly Toads, though, I'm not sure - I've simply been using your basic Fowler's Toads that are common pretty much anywhere you look. When it comes to amphibians, and particularly toads/frogs, I have little or no knowledge - but I can tell you that all toads/frogs are toxic (though potentcy is variable).
If you do try to feed that little guy to your scorp or your T (and I'm not say you should!!!!!!) please post your results (and include species names!). I, for one, am very interested.