I've kept a bunch of Hoyas in direct light in a southern window before and the only one that didn't like it was bella Luis Bois (the variegated portions of its leaves got a little burnt). And even that one probably wouldn't have burnt if I had given it more time to adapt, since Renee keeps hers in pretty high light conditions.

The truth is, Hoyas come from a lot of different habitats. For example, a number of Hoya australis varieties grow on rock faces in full sun. Others grow in a more sheltered canopy environment. So, there's just not a "one fits all" lighting rule.

That said, I think if you up the light gradually, most will be fine in your unshaded south window. Just keep an eye on the situation. That's what I would do.

See the post below for more pics. ^_^

Here is a link that might be useful: Post by tropicbreeze on Hoya habitats

Can I keep a Carnosa 'Krimson Q' where it gets a little morning sun, but then goes into sunlight through greenhouse wall the rest of the day?

Thank you for the explanation. I will start gradually introducing sunlight to the plants.