So for your first question, yelling "Stop Thief" loudly at the thief is perfectly legal (Like Yelling "Fire" in a crowded theater, it's legal if the theater is infact, on fire... the quote implied that it was a prankster who drew amusement from the reaction of the people who took him seriously). This may alert store employees, who have a specific kind of Citizens Arrest Power known as "Shopkeeper's Privilege" and is a reduced liability compared to Citizens Arrest. This is also pretty basic self defense agains people who are not engaged in any physical action against you but are starting to scare you. Suddenly shouting draws attention to you and discourages them from their behavior "I SAID DON'T TOUCH ME" being suddenly shouted in a Wal Mart is going to get notices from the immediate crowd drawn to you, and by poximate location, the person who is making you uneasy.
For your second question, maybe, it depends... since the cop is clearly chasing the guy, you meet the qualification to use non-deadly force in stopping the criminal (i.e. you are witnessing a person committing a crime, in this case, resisting arrest or persuit) and it would certainly be reasonably non-deadly force if you were to trip, grab, or push the fleeing suspect. Citizens Arrest doesn't specify a minimum time to qualify, so if the officer is seconds behind, and you detain the criminal in this action until the gap is closed, it still counts.