Can I specify only a subscribe key in iOS App?

Yes, for a subscriber iOS client connection (if you only need to subscribe but not to publish), use placeholders for the secret and publish keys:

    PNConfiguration *myConfig = [PNConfiguration 
            configurationWithPublishKey:@"none" 
            subscribeKey:@"sub-c-d0dbe894-2a07-11e2-945b-7d0e10df7..."  
            secretKey:@"none"]; 

    [PubNub setConfiguration:myConfig];