First off I'm terrible at guessing size, so I won't even guess... i'll use small or big pretty much...

I have 2 small oscars, 2 convicts (which are a little bigger), and a jack (which is quite a bit bigger).

I want to put them in the same tank but I wasn't sure if the jack would beat up the convicts and oscars... or if they would all play nicely, or at least "cichlid nicely"...

well if it's a huge tank...like a couple hundred gallons, they might play nice but for most smaller tanks, I'd say it'd be a no-go. Others might disagree but I wouldn't do it personally
mike

I agree with mpp, think you will end up with a warzone in your tank

Yep.

To put it in perspective - just the 2 oscars alone should probably have 150 gallons to themselves. Way too many fish!

I had two oscars and a jack FOREVER ago, in my 30 gallon tank, and they got along fine, and had plenty of room to swim and stuff. They eventually got pretty big so I took them to aqua pets, cause I didn't have enough money to buy a bigger tank. I got new tetras and an angelfish and a couple other guys, and I still have that tank going now.

Now I have another 30 gallon tank, and I was gonna try to redo the same thing I had (I have a 55 gallon tank in waiting also for when they get bigger...). I don't understand why I would need 150 gallons for 2 oscars. I realize they get big, but I never understood the... you need to put them in the tank they're gonna be in forever thing.

Perhaps I'll trade them in then for something a little smaller

It really depends on the size and growth rates of the fish. If given enough room, Oscars can grow to be tank busting 5 pounders in a couple -three years from time of purchase. Keeping them in a smaller tank will look like it's delaying their growth but its most likely stunting. Can you keep Oscars in a 30? Sure as long as they're small but it's better for the fish to give it adequate room for how big it will be at maturity, not infancy or the juvenile stages.
good luck
mike

how about discus in a 30 instead? I have a a fish book that says for a community of discus, that a 30 is good, but it seems like everyone seems to require more...

of course more is better... but would 30 be ok for discus?

Gary is the local discus expert, let's get his opinion. I'd say a 30 is fine for small ones but I'd still probably want a bigger tank for them as they mature. Granted they don't get nearly as big as oscars but they can get up to salad plate size.
mike

How tall is the tank ?

salad plate size!? wow, that's HUGE... or I've been using the wrong plates for salad...

I think it's 24" tall, although it may only be 18... the depth is 6 inches less, either way... 36" across.

I have a similar tank--it's a 30 gallon long. I have never kept discus, but I can imagine a salad plate floating along in there, and it seems a bit shallow IMO.

I personaly would not use a tank any smaller than 24" in height as a bare minimum for disscus, as they are a deep bodied fish so requiring a deeper tank

I agree with clunk, mpp and arb. In fact I just upgraded my 50 gallon to a 75 gallon so I can house more than 2 at adult size.