Gary,
Yep, just do the modules at whichever school suits you for whatever reason e.g. cost/location/reputation... Once you've done your PPL you can the begin the ground school. It's fairly demanding and getting the ATPL writtens out the way before you complete any more expensive flight training is prudent in case you decide to stop along the way. I've seen people drop out despite giving it the big one about how keen they are at the start!!
Cheers,
Biscuit
PS Getting the PPL in 21 days doesn't always happen so be prepared to finish up when you get back if you run out of time. I assume you'd be going State side? When I did my PPL at a well known US school, not many completed within 21 days. However, it was during the summer and the weather in Florida isn't always as perfect as they make out.