thats not a red tailed shark lol its a red finned shark (or a rainbow shark). its less aggressive than a red tailed shark but i can assure you, it will most probably tear the betta to peices in time
they might be getting on now, but that can easily change with fighters, i have seen it happen way to often, after a few months they suddenly turn.
keep an eye on the fighters tail, looks a little battered to me
I've heard of fighters getting on well in all sorts of community tanks, I wouldn't be worried about the shark in your tank myself, I'd be more worried about your big blue gourami.