benefits of disabling built-in apps on zte z777 On a ZTE Z777 phone, would I gain USABLE app space and storage by disabling built-in
 apps that I don't use ?

Also, I have read on this site that some built-in apps may or may not cause misbehaving of other things.   How to know which apps are ok to disable ?
On a ZTE Z777 phone, would I gain [usable] app space and storage by disabling built-in apps that I don't use ?


Yes and No. Yes, because if the system app1 has been updated then it would be storing the update (apk) under userdata2 partition (/data), which does consume good amount of space. Disabling such an app would get rid of the update (apk) as well as the user specific data stored under userdata partition. No, because not all the system apps get an update or stores considerable amount of data under userdata. However, if you disable such an app that miniscule amount of data would still be deleted, hence freeing up some space.


  [Some] built-in apps may or may not cause misbehaving of other things. How to know which apps are [okay] to disable?


You don't, although many Google apps (shipped with device) have proven to be safe to be disabled. Consider this rule: if you've a custom recovery installed or ready to sacrifice your data then you can risk going berserk by disabling whatsoever you want (provided that you can, as an ordinary user). Otherwise, play safe i.e. do a web search to reach what system apps have folks found safe to be disabled. 



1 System apps are primarily saved inside system partition. You cannot remove them from system partition without root access.
2 All user installed apps, updates to system apps and all apps' data is stored in userdata partition. It is the space here which counts as usable.