Lep is right but as he hints those combinations can be done... I suggest visiting the species sub-forums and finding threads with any combos you have in mind and reading as many outcomes as possible.
Personally I have done all three of those to various degrees of success...
Gars w/ Rays: Compatibility is fine, but the gars sometimes go after food the ray has and end up nipping the rays disc. This can end up in slight damage to the ray as well as stress. The worst case scenario is the gar being stung, but I have never seen this occur.
Gars w/ Arowana: This is a more common combination. I find if you put a lot of effort into timing the growth rates you can keep these fish together with minimal aggression. Silver arowana's can be more aggressive than blacks. Problems may arise during feeding when the arowana tries taking food from the gar resulting in snout damage or a very skittish gar that may end up crashing into the glass and breaking its spine. A very iffy combination, just as Lep said.
Gars w/ Catfish: Depends entirely on the catfish, to the point that there's really not even much to say. I've kept big pims with gar before and had no issues, but bigger cats like RTC would likely pose a problem to the gar.
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So realize anything is sort of a risk, but can be done if enough preparation is done.
