I would say in that size tank you have a fair chance of it working with the Oscar and the GT. Get them both young, the Oscar will grow faster, but not by much in my experience. You will obviously need a decor to allow for plenty of seperate areas.
I would add that as you know it will come down to the individual temprement of the fish. Up until last week I had no problems with my O, GT and Sev, My severum near 8" was the boss of the tank, the GT would always be cautious around him, not that there was any aggression from the Sev, but he would break up any problems with the other fish, then suddenley and literally overnight the GT turned and starting to terrorise the Severum in particular and the Oscar (who is still small at 3-4") the GT was around 6" which is when they start to become aggressive. I decided to rehome the GT.
Its always a trial and error with these size/aggressive cichlids, just be on the watch for any problems.
I would also say get the ghost knife as large as you can, they are slow growing, and you may need to target feed for a while, as they can be slow to get to food, particuarly with greedy large cichlids about, but other than that Ive no problems with mine, there have been reports of large Oscars enjoying a game of toss the ghost knife around by the tail, but by and large they should be fine.
Cant comment on the shovelnose, know nothing about them
Have you considered instead of silver dollars as dithers, maybe Bala sharks or tinfoil barbs?
- so long as they're fairly young (although make sure the oscar is smaller than the GT and the sev as it will grow much quicker!)
but your best bet will be to get juvies and all should be ok.
