is this normal behavior? (video)

Could possibly territorial aggression by the clicking sounds, the gar was smart enough to move away just keep monitoring the gar and loaches. BTW tank looks great

The only CL's I've owned are the ones I currently have and they are big babies when it comes to tankmates. All 3 just zip around the tank or play in the return current from my canister filter. They were in with to florida gars for about 3 weeks too with no problems.

The twitching concerns me...I'm having the same problem with one of my sunfish currently. I would look into possible parasite treatment as I've seen my sunnie flashing on the driftwood too. I currently tried prazi-pro for gill fluke since there are no visible signs on him. Gar are kind of prone to parasites so it couldn't hurt to try. His obsession with the powerhead could be pointing to a breathing problem due to parasites, hence why hes swimming into a current.

When your gar appears to be snapping its head it may be resetting its jaw, which is what I was told by my LFS a couple years ago.

Gars are sometimes odd when introduced into a new environment, one of my gars now swam against one corner of glass for a month only stopping to eat, he swims normally now. My first gar swam in the exact same circle, slowly, but all the time but also eventually stopped. Also, an alligator gar I had swam in a powerhead like that too, but so close to it and also eventually stopped.

As long as your gars are eating, and don't show signs of disease, giving them plenty of time to settle in is my advice.

Also, if your gar is being attacked, then you might just want to separate it from the clowns for now. But your gar could start snapping back or get big enough to eat them.