ok, will lay it out for you... Read the stickies on this forum before you even think about getting a ray. There is good advice there and alot to learn from.
Second, an oscar in my opinion is not a good tank mate. If you think adding sand is a good idea to give the ray a place to hide, then that rt there answers your question. The less stress to any fish the better. Longer life and health.. If your fix is to add sand to give him a place to hide, then that not the fix. Like I said though, Oscars and rays in my opinion are not a good idea.
Third, Even if you get the smallest ray you can find, a 75 gallon is not a good size for a ray. One, rays grow. 75 gallon is not big enough for a ray to move around and promote good health, nor is it big enough to hold the amount of Bio-Load a ray puts off.
If you want a ray, get at least a 150 gallon tank to start off with. Small pups will do fine in a 150 for at least a year or so. But you will need to upgrade in the future. (check craigslist for sales on a good tank)
Tank size is important when deciding to get a ray. Gallons as well as how the tank is layed out is the things to watch for. The wider the better in my opinion.. If you get a small pup to start, a 150 will work for now, but I would watch for a larger tank.. Sometimes best to just get a larger/wider one to start with instead of upgrading.. But those tanks are also hard to find.. Esp at the rt price..
No Oscars in my opinion.
Regular water changes is a must.
Good filtration is a must.
Sand is the best in a ray tank since it is less harmful to a rays disk than aquarium rocks.. and also just incase they do get scared, gives them a place to hide.
(sand can get into the filters though, so make sure your intakes are at leasy 3 to 4 inches above the sand..
With this advice, you would be ok. BUT to answer your questions...
NO, I dont agree with your tank mates and having a ray.
NOR should you put a ray (ever) in a 75 gallon tank for periods of time..
