Actually saw my RTCxTSN and gar laying together last night during a wc. Not close to each other, my gar was literally laying on top of my catfish. They are best buds, the catfish chases fish toward the gar when i throw in feeders
There are a few things that you need to consider before attempting said combination(s):
How big is the gar?
How big are the fish(es) that you wish to house with the gar?
What is the temperament of the particular fish(es) that you want to house with the gar?
The main concern with the RTC is if it can eat the gar and readily outcompete it for food which is why many people recommend against keeping them with gars although the combination seems to work well if they are given enough space and are both at their maxiumum captive size.
As for the clown knife, I would be afraid that the gar would shred it (or maybe even vice versa), but, once again, it's a combination that has worked for many people with varying degrees of success.
I'm not entirely sure on the snakehead, so I would listen to those who have attempted this combination before and/or those who have experience with snakeheads.