My recommendation would be to go with the Oscar as the "showcase fish" and build around him (but buy the Oscar last).
Here's what I would do with a 180G
5 Standard Silver Dollars
7 or 8 Clown Loaches
1 Pink Tail Chalceus
1 Tiger Oscar
I would buy the Silver Dollars first (mid-level swimmer)
Two weeks later, get the Clown Loaches (lower level swimmer)
Two weeks later get the Pink Tail Chalceus (upper level swimmer)
Grow the Silver Dollars, the Loaches and Chalceus out for a bit and let them get settled in the tank. After they have put on a little size and have become comfortable in the tank, get the Tiger Oscar.
If you were to add the Oscar first, there is a chance he could be territorial and act aggressively towards any additions to the tank. The Oscar will also grow faster than the other fish, so it's a good idea to give the others a "head start." Adding the fish in the order I mentioned will give you the best odds of sucess.
A picture is worth a thousand words...here's my old 270G 6x3x2.
