No gesneriad that I know of will tolerate a full southern exposure. Certainly some will appreciate more light, like Aeschynanthus and Columnea, but neither would survive the amount of light you're talking about. Is it possible to put a sheer curtain in the window? This is usually sufficient to allow the plants to do well.

Thanks.. Sheer curtain isn't an option, since it's a storefront/should be open to the public view. :\

In winter, my streps don't mind unshaded southern exposure at all. I wouldn't want to try it past Spring though...too hot.

Depending on the exact situation simply moving the plants further away from the glass can solve the too much light problem, but it might start taking away from your retail space then.