I personally would not risk it. I would never keep anything with lovebirds except lovebirds. They have a beak that is not to be messed with- one swipe and your cockatiels lose a toe, beak, even an eye. Lovebirds can be quite aggressive and territorial. Despite being bigger, cockatiels are a much gentler species and the lovebirds will likely walk all over them until they have had enough. If a squabble breaks out I can guarantee the cockatiels will come off second best. That cage to me doesn't sound like it will big to allow them their own space, sorry.
I must admit I am quite surprised that a petstore would house them together. Makes me wonder how many of them they lose... The other possibility is that they have been housed together from a young age. If they are raised together then it could eliminate some of the problems. I used to let my tiel play with my sister's lovie until it was about 6mnths old. Then she was more confident and no longer intimidated by my bird and I worried too much about his safety.