Well if it is on Youtube it must be correct.... Youtube after all is always correct and a great source of detailed information........ Right? No need to have anything like input of those experienced in such things involved in these things.. It's on youtube so it just has to work and will work... So yeah just go right on and try that out... Youtube vids say it's all good so go for that if it is your choice.
^^^While he may have gotten up from the wrong side of the bed today

, he is the expert.. @OP: please listen to him
I have 2 FL gars and 1 Silver Arowana who have been in the same tanks for about 1.5 years. I even had 2 Alligator Gars with them for 4 months before the Gator Gars grew HUGE and I got rid of them.
I grew out the Gars and the Arowana in separate tanks from 4inch to about 10-12 inches, before putting them all together.
Despite the relative success I have had keeping Gar with otherwise incompatible fish (Silver Aro, Jaguar, Oscars) , my experience is an EXCEPTION.
You can try your luck and try this combination,
but the odds are against you.
Also, both the Arowana and Gator Gars need a tank much much much larger than 75G.
My Silver Aro went from 4 inches to 26 inches in 20 months (It'll be exactly 2 years on Dec-9-2011)
My Gator Gars were 4 inches when I bought them on Aug-8-2010. They were 18" when I got rid of them in late Nov 2010.
So.. tl;dr: VERY bad idea!!!