When George and Lennie first meet the Boss, George does all the talking. The Boss presses them, asking why Lennie doesn't answer for himself. George says that Lennie isn't bright but he is a good worker. The Boss becomes suspicious that George might be taking some of Lennie's pay. George assures the Boss that he is just looking after Lennie. The Boss is still suspicious but allows them to work:
All right. But don't try to put nothing over, 'cause you can't get away with nothing. I seen wise guys before.
Slim is less suspicious than the Boss. But Slim is not naive either. In his simplistic yet sage-like way, he makes a general comment that most men don't travel together because everyone is scared (or suspicious of) each other. This is exactly why the Boss is suspicious of Lennie and George. In the world of itinerant ranching, there seems to have been an "every man for himself" attitude. George and Lennie are exceptions to that generalization. Slim has certainly seen "wise guys" before but he is not as paranoid as the Boss.