Would like to comment that a certain custom entity (without a runtime_identifier) was considered to be a minecraft:cat by commands (\kill @e[type=!player] would kill it and say "Killed Cat" and \kill @e[type=cat] would kill it)
Oddly enough, this only affects the FIRST sign you set down after loading a world. At least on mine (Win 10)
In beta 1.10.0.3, custom entity identifiers still seem to be unable to be used in type selectors, though I do not know if changes to addons has somehow caused the custom entity I'm testing with to become somehow partially invalid.
/summon custom:entity
custom:entity is listed as a suggestion and the command works (poor guy's invisible, though, despite otherwise functioning normally and making noise)
/kill @e[type=custom:entity]
custom:entity is not listed as a suggestion, command fails with syntax error
/kill @e[type=!player]
kills entity with the output "Killed Unknown"