How Long Can I Use Temp Crown on my Back Tooth?

The lab should be able to deal with your concern. Explain it the way you wrote this letter and everything should work out.

Your teeth shouldn't move unless your bite contact is heavy. If it is, the light/hard tapping continuously over time may move one or both. Also if gaps are left inbetween any of the teeth, the space will allow some movement.

It is sometimes advantageous to splint teeth for strength but it should only be done if necessary.  You can floss splinted teeth but you must use a floss threader.  The temporaries look oversized, make sure you are happy with how the permanent crowns look before they are cemented.