brich 999 .......... the answer to tank size can be found here:
http://www.monsterfishkeepers.com/forums/showthread.php?402217-The-Hypocrite-thread-mbu
That thread debunks the "1000 gallon min" that you constantly read on the interweb.
Water quality is just as important as tank size, probably much more so. Your tank is plenty big enough for a single adult mbu, as long as you are willing to keep up with the water quality as the fish matures.
As far as the cat & gar, obviously the gar is out of the question, and personally I wouldn't keep a cat that has the potential to do harm either.
http://www.monsterfishkeepers.com/forums/showthread.php?402217-The-Hypocrite-thread-mbu
That thread debunks the "1000 gallon min" that you constantly read on the interweb.
Water quality is just as important as tank size, probably much more so. Your tank is plenty big enough for a single adult mbu, as long as you are willing to keep up with the water quality as the fish matures.
As far as the cat & gar, obviously the gar is out of the question, and personally I wouldn't keep a cat that has the potential to do harm either.