61.156 applies only to those applying to take the knowledge ("written") test after July 31, 2014. Until August 1, 2014, the "old rules" apply, i.e., for all intents and purposes, 61.156 doesn't exist and can be ignored. Further, the new requirements only apply to taking the written, not the practical test, so if you take and pass the written before August 1, 2014, you can still use that written to get your ATP for 24 calendar months (i.e., up through July 31, 2016).
Note also that on August 1, 2014, the ATP written (currently one test for all ATP-Airplane ratings) will be split into separate SE/ME versions, and if you take the test on or after that date, you'll need the ME version to get your ATP-ME rating. However, even after July 31, 2014, you'll still be able to take the new ATP-SE written and get your ATP-SE without going meeting the 61.156 requirements (which apply only to taking the not-yet-effective ATP-ME written).
One other point -- it appears from the regulations that if you have an ATP-SE-only pilot certificate on July 31, 2014 (based on having taken the current non-class-specific ATP-Airplane written), you would not have to take the ATP-ME written that comes into existence on August 1, 2014 (and thus will not have to meet the 61.156 requirements) no matter how long you wait to add the ME rating. So, if those new 61.156 rules will be a problem for you, and you can get your ATP in a single now (even a 172 will do), you might want to get on that bandwagon before the end of July.
References: 61.153(e), 61.156.
