You guys crack me up. The chances of breaking off a heel/horn using bottom tools in the hardy is about as high as winning the lottery. The only case this is going to happen on one of these anvils is if you use a jackhammer to strike and there's a flaw in the casting. The two very large anvils the OP is considering buying, are large enough for work 10 times bigger than he's likely ever going to do.
Also, you hot fit stock to your hardy. Only in modern times do we buy pre-made, sloppy fitting bottom tools that bounce around in their holes. If you've got a bunch of stock you want to use, get it hot, taper it a bit, and pound it in the hole with a sledge, then push it back out the other side, dress it a bit, and mark the direction it fits in the hole. Of course, don't be afraid to dress, or resize your hardy hole if you feel there's a good reason. Tools are meant to be modified to their user's purpose. Not sure why there's this culture of assuming everything is "made that way for a reason, and even if I don't understand why, better not screw it up".
Anvil edges are sharp usually, but should be dressed to the radius you find appropriate. Leave a small section sharp if need be, but it's inefficient to leave your working edges sharp, the whole point of having these "dies" is to move metal productively. Tapers are performed on near and far face radiuses, if they're sharp, they have the potential to leave cold shuts, and are very difficult to planish.
Hardy and pritchel holes should be dressed also, they're not done at all, or roughed in, because it's a subjective process that's labor intensive. Each smith should know how he wants these shoulders dressed.
A vast majority of work in the recent past (before the near death, and rebirth of smithing) were done using top and bottom tools, because of the potential for symmetry/asymmetry. Admittedly though, every smith out there had a striker or strikers to work with.
