You shouldn't need to download another app to change the ringtones, you should be able to go to to Settings -> Sound & Notification -> Phone ringtone. It really shouldn't change the ringtones by itself, either.
If you're running out of storage space, try installing
SD Maid to clear out some space.
As for the different volume types, that's just an Android thing. Having the volume levels separate is good in theory, but can be annoying sometimes (for example, you might try changing the Media volume only to have the Ringtone volume change instead). I think this has been fixed in Android Marshmallow, but I can't confirm this myself as my phone is running Lollipop.