When is contact allowed ?

I was advised players are allowed to push, elbow and shoulder their opponents if it doesn't interfere with the course of the game, is this correct ?

you are not allowed to contact or obstruct a player on purpose in order to get better position or the ball.  there is a rule called "fair contest" where 2 players are allowed (while attempting to get the ball) unintentionally contact each other, but it is up to the disgrestion of the umpire as to whether they believe it was intentional or fair contest. 

Technically speaking "no contact is allowed with an opponent in such a way that interferes with the play of that opponent" (abbrev. Rule 17.1) so if contact occurs that does not interfere with the play it isn't generally penalised.  It can however be dealth with under Rule 20. as rough or dangerous play if excessive and a warning given etc.  Umpires are encouraged to allow fair contest for the ball which also relates to the situation if two players come into contact with each other in attempting to play the ball then play continues with no "toss up for simultaneous contact" that used to happen years ago.