Hi Prabakaran,
Are both the beef and chicken in your first question cooked or uncooked?
As for your other two questions I'd stay well clear from storing cooked and uncooked on the same level, even though covered, purely because of the possibility of the film wrapping being contaminated by the raw meat juices and becoming a potential vehicle for bacteria to travel on. And there's no real way of telling when a piece of food is contaminated enough to cause food poisoning- that's where shelf life tests at laboratories and HACCP come into play.
If in doubt, throw it out.
Regards,
Andy