Synspillum IMO/E aren't as agressive as flowerhorn. All the Vieja I've known have been very chill but "don't **** with me" fish. If the Syn's the smallest he'll probably hide in the shadows until he gets bigger. This is a really subjective question though, you cant say for sure. I would not mix oscar with a flowerhorn though.
Depending on your fish sizes and your flowehorns temperment you could pull this off for maybe a year. The parrots would have to be much bigger than the FH though, as some FH will just start murdering in the night.
You might be able to do it for longer without the flowerhorn though. FH's's th sticky wicket in this situation.
What all of these fish do you have already, and which ones are you considering getting?
That would be important, as you gotta include th ones you got