True most of the time it does, sometimes I feel we could do with fewer playmakers and a bit more grunt in the pack but I can't really complain at 6 titles in 10 years

Yeah youve got teams like Catalans playing Bosc on the wing for at part of the game, some teams have full backs coming in as halves, some teams have hookers, some teams use halves as extra hookers, some teams use back rowers as extra or stand-in halves. It all depends on the system the club wants to use.
From my albeit limited watching of the NRL I'd suggest that the halves in the NRL seem to play a more similar role to each other than the halves in SL.
Is that why they find it easier to fit into a national team?

It depends, the number on the back (6 or 7, not this squad number malarkey), doesn't mean much, Widdop will be the dominant half for the Dragons this yr whether he'll be wearing 6 or 7. Thurston is playing the same as he was when wearing number 7 for North Queensland, still trying to do too much. Ray Thompson and Robert Lui have to step in there and tell him he's doing to much, and let them take some control. Watched some of the match against the Penrith Panthers last yr, he's putting some of his team mates off.

Lewis used to be the dominant half, Johns used to be the dominant half, yet as you know one wore 6 the other 7.

A scrum half is more likely to cause a fight.

Great point it always seems to be a scrum half who starts fights.
(Also hookers do obviously)

One Usually has 6 on his back and the other has 7.

I guess it depends what the coach wants from their halves. Sydney Roosters play with Maloney on the left, Pearce on the right, and they share the play-making responsibility. North Queensland Cowboys have Thurston who basically takes both roles upon himself and his halfback partner Robert Lui just plays off him. Canterbury Bulldogs use their half-backs as second/third receivers and let their prop forwards take first receiver. It really does vary with every team. The definition of a stand off or scrum half is really want the coach wants from them.

I prefer the title 5/8th.