I don't have any Hermann's tortoises, YET... but I'll try to answer in a general tortoise sense. In general I wouldn't make flowers any more than 5-10% of any Testudo sp. diet. Those numbers are just a generality. They aren't going to just suddenly drop dead if you go as high as 11% one day. I use flowers as an occasional treat and supplement to their regular diet.
There are literally dozens of appropriate species, but be care of pesticides and chemicals if you did not grow them your self. I like hibiscus, roses, rose of sharon, carnation, blue hibiscus, clover flowers, and dandelion, to name a few.
Like Tom says, don't let flowers be a big part of their diet, but some is fine...and will be accepted w/ open jaws! Mine seem to prefer yellow ones, best, so that includes dandelions, hibiscus, Chrysanthemums, etc.