We developed apps in Shopify and now we want to build the same apps as extensions in Magento. Which edition of Magento should we start with?
Do we need to develop an extension to Magento enterprise edition and Magento 1.9 separately or it is the same? Is Magento 1.9 and earlier versions such as Magento 1.8 are the same?
I'm glad you're interested in Magento platform.
There are huge differences between Community and Enterprise editions, but the core is the same. So, depending on your extension, you might have to do some additional work for client which are using Magento Enterprise (to solve compatibility with full page cache, for example).
Also, keep in mind that Magento released version 2.x one year and a half ago, so it might be better for you to start with Magento 2.x immediately.