So, if these deaths are disease related (such as if the butterfly or blennies died from ich or velvet - velvet would be my personal guess in this case, but I'm definitely not a disease expert), then waiting just one week means the disease is probably still in the tank and may be infecting any other fish you add.
To be safe, you'd want to wait at least 45 days with the tank at 81F (~27.2C); 60-76 days is typically recommended to be safe; waiting this long ensures that the disease has died out in the tank and can't infect new fish.
That said, if you're struggling with disease (which maybe one of the
#fishmedic can tell us if you are), then you'd probably want to figure out a way to quarantine and treat any new fish so they don't just die from disease and re-infect your tank.
Some people are able to keep diseased fish alive by keeping them healthy and not stressed, but I don't know how well those methods work against velvet, and I don't think you'd be able to keep a butterfly from being too stressed when it's in a 10 gallon tank regardless - blennies maybe, but not a butterflyfish.
So, personally, I'd assume this is disease-related, I'd set up a cheap quarantine tank if you can get a hold of some fish meds, and I'd pick some small fish to keep.