Avoid monolithic applications -> architecture doubts

Hi,

     We are developing an APP and we like to make it as modular as possible, the reasons?      - Isolate functionality      - Be able to work with different versions, for example module1 with rails 3.0.1, module2 with 3.0.2, module 3 trough WS with an external APP, ....      - Avoid monolithic applications, we think they are hard to maintain

     Our first idea was to develop one APP for each module and we where discussing how to connect them.      - Creating a "monolithic gem" with all the shared services (modules, routes, views), and erasing the functionality from the original APP      - Creating a "monolithic plugin" same method... but in this case we think will have to repeat the functionality between de APP's and the plugin.      - Communications trough web services between APP's

     Which problems are we encountering?      - Some modules need to be almost everywhere, for example in a medical APP we need the administrative module (name, gender, birth, paying info), almost everywhere (hospital, emergency, radiology, ...)      - In the gem and plugin, we have doubts how to synchronize the migrations, images, css, updated everywhere.

     I like to know which approach you consider the best for this architecture, problems that you encountered or any advice / opinion.

     Thanks,