Ofc you could run heretical sisters! I think that is an awesome idea, its something thats skimmed across my mind a few times. Chaos is so open to conversions its unreal, as chaos is in effect simply an idea rather than a race or a thing it can be applied to almost anything.
The best bit about it, is you could run them either way. You could run them as corrupted sisters using
SoB rules, or if modelled the other way you could run as chaos marines. as in reality as long as the opponent knows what model it represents the game play is totally unaffected by the model aesetics (the only thing to be careful of here is sizing, but typically thats not an issue either). I love the idea of a canoness as a
DP, I personally (while still also loving big monsterous
DP's) think that
DP's look sick as human sized (or slightly larger) models, representing that through deamonhood they have gained powers that simply turn their human form into an absolute killing machine.
I say why not

If you have the time to put in then I see no problems. Just dont buy em and do nothing with em!
Edit: oh and in terms of Toughness and
Str, you could literally just say well they are more agile than standard marines which makes up for their lesser toughness and that as its chaos they are blessed with inhuman strength

Chaos is literally that easy as it technically isnt limited by any physical factors.