Can I buy a Samsung Propel from AT&T with a 2 year contract, then cancel...

You cannot cancel your AT&T contract within 30 days of agreement. If you are willing to do so, you may be need to pay heavy charges. Before using a Tmobile SIM with the Samsung Propel, you need to unlock the device. AT&T won't provide you the unlock code unless the contract is finished. But you can get the unlock code from any trusted vendor online. For example, you can get the unlock code from Unlocking4U.Com at affordable cost with easy unlocking guide.
Regarding using your plans on certain days alone totally depends with your network provider. You must contact their customer care and seek for a solution. Good luck.