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You should look at this : Which combination of ROS/Gazebo versions to use.

Under the default Gazebo version for each ros distribution section it states :

Melodic

ROS Melodic hosts or use the 9.x version of Gazebo. For a fully-integrated ROS system, we recommend using the 9.x version of Gazebo. The way to proceed is just to use the ROS repository (it will automatically install gazebo9) and do not use the osrfoundation repository.

This answer your second question, the default version for melodic is Gazebo 9 not 8.

Kinetic

ROS Kinetic hosts or use the 7.x version of Gazebo. For a fully-integrated ROS system, we recommend using the 7.x version of Gazebo. The way to proceed is just to use the ROS repository (it will automatically install gazebo7) and do not use the osrfoundation repository.

There is also a section about using another Gazebo version than the default one, you can do that by installing from the osrfoundation repository, but don't miss the warning :

Warning!: Using this option, you won't be able to use any ROS Ubuntu package related to Gazebo from ROS deb repository. The equivalent of gazebo_ros_pkgs can be installed from debian packages, but all other software (such as turtlebot_gazebo) must be built from source.

That means that it could work but you would have to build all the gazebo related package from source and if a package has a compatibility issue you might not be able to use it at all.

You should look at this : Which combination of ROS/Gazebo versions to use.

Under the default Gazebo version for each ros distribution section it states :

Melodic

 
Melodic
 
ROS Melodic hosts or use the 9.x 9.x
  version of Gazebo. For a a
  fully-integrated ROS system, we we
  recommend using the 9.x version of of
  Gazebo. The way to proceed is just to to
  use the ROS repository (it will will
  automatically install gazebo9) and do do
  not use the osrfoundation repository.

This answer your second question, the default version for melodic is Gazebo 9 not 8.

Kinetic

 
Kinetic
 
ROS Kinetic hosts or use the 7.x 7.x
  version of Gazebo. For a a
  fully-integrated ROS system, we we
  recommend using the 7.x version of of
  Gazebo. The way to proceed is just to to
  use the ROS repository (it will will
  automatically install gazebo7) and do do
  not use the osrfoundation repository.

There is also a section about using another Gazebo version than the default one, you can do that by installing from the osrfoundation repository, but don't miss the warning :

 
Warning!: Using this option, you won't won't
  be able to use any ROS Ubuntu package package
  related to Gazebo from ROS deb deb
  repository. The equivalent of of
  gazebo_ros_pkgs can be installed from from
  debian packages, but all other other
  software (such as turtlebot_gazebo) turtlebot_gazebo)
  must be built from source.

That means that it could work but you would have to build all the gazebo related package from source and if a package has a compatibility issue you might not be able to use it at all.