Lets look at your options, you have quite a few fish already, including 2 Fuzzy dwarf (D.
Brachypterus).
If you want something different you could look at the Fu machu (D.
Biocellatus) but I'm sure you know that they are difficult to keep and can be a PITA, but they are small enough to probably add.
Then you've got the Zebra (D.
Zebra) but these can be aggressive to the same species and IMO are very similar to your Fuzzy.
So that brings you to the
Pterois species which are larger, (but my favorites

) I think you can discount the Volitan, as most of your fish would make a tasty meal

Have a look at P.
antennata, max. length around 7", if you can get hold of a juvie even better, they are good aquarium fish, they may eat smaller fish, so you have to weigh up the odds
The only other one which you might get away with is P.
Russelli it is the smaller volitan species but can still grow to around 11".
I would personally say go with a juvie P.
antennata 