Hi, I have a 4ft tank in which I have 2 oscars, 3 terrapins and 1 large pleco catfish.
I got all of them at the same time when they were all small, now live in a very happy tank oscars love to steal the terrapins food as they are much quicker!
Have had a few minor scuffles but no one has got hurt yet, been going for about 9 months.
But I would only recommend doing it this way, if you put in small oscars with big turtles or small turtles with big oscars then someone is going to get eaten!
Water quality is an issue as everyone in the tank could crap for England, especially now they are all getting a fair bit bigger.
I do a 50% water change every week and have a fairly industrial filter system, but as I said above no issues so far.
Hope that helps.