We shouldn't be changing the name to Running apps. Especially in like the web services. This breaks backwards compatibility.
Can you clarify your comment "because maximum active applications is used in documentation for number of running + queued applications."? Yes there can time between when an application is made active vs when the AM gets started - waiting for a container. Is that what you are referring to? I don't think with your patch this has changed, correct?
I believe the main reason it was a percent is because AM's could potentially use different amount of memory per AM (YARN-276 related). So I could have 2 AM's that if using the minimum allocation size would run fine but then if I have 2, one that say needs 1G and the other uses 20G, then your queue is full and everything gets deadlocked. Hence why in that case it would be better to use a % of resources.
I can understand your scenario though. I would say if it was just one you could set the percent really low like (0.0001) because it always give you atleast one, but if you have cases where you want 2 or 3 that doesn't work so well.
You also need to update the capacity scheduler docs about the config.