First, Once USCIS receives the H-1B Transfer petition, you are good to start with the new company. Second, you do not have to wait for the approval or the receipt notice. Some prefer joining upon receipt of the receipt notice. Third, RFE is only issued for the issues relating to the date prior to the filing. Therefore, the anticipated RFE, will not have such inquiry. In case they ask for pay stubs, which is highly unlikely, after the date of filing, you can submit pay stubs from the new company. Good Luck.