You can run a mixed reef, but there are issues...

The things the articles discuss are all real, but it doesn't mean it's going to be a problem, necessarily.

There's several things here; Soft coral tanks tend to be "dirtier" than the pristine environments that hard corals need, so there's one incompatibility, and soft corals don't tend to do so well in pristine environments, but if you maintain a middle of the road tank, with good parameters, then most softies will be fine, and most stony corals will be fine too.

Soft corals also can partake in chemical warfare, where stony corals tend to just dissolve each other in physical fights with neighbours, and so when soft corals release terpenoid compounds which inhibit the growth of neighbouring corals in the wild, in a tank obviously the toxins affect every coral in the tank, but running activated carbon and water changes will stop that being too big a deal.

Anyway, read this. Anything from reefkeeping.com is usually written by Dr Rrandy Holmes-Farley and co who are very well respected in the field.

Soft Corals: Corals That Paved the Way for Modern Day Reefkeeping by Mark van der Wal - Reefkeeping.com