merged map flickers on Rviz

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That's what I think aswell, but the init_pos-es of the robots are required, I can't pass them all at once. That made me think that multiple instances were required

<robot_namespace>/map_merge is a namespace. The map_merge there is not a node. It's just the namespace the map_merge node reads the parameters related to <robot_namespace> from.

I am sorry, but I didn't understand what you mean. If there are multiple different namespaces (robot1, robot2, robotN) how can map_merge get all the parameters it needs from all of them? Eg the init_poses, the current namespace and so on

From the wiki page:

multirobot_map_merge finds robot maps dynamically and new robots can be added to system at any time. To make this dynamic behaviour possible there are some constrains placed on robots. First all robots must publish map under <robot_namespace>/map, where topic name (map) is configurable, but must be same for all robots. For each robot <robot_namespace> will be of cause different.

The node knows how to "scan" for new robots, provided they've properly namespaced their topics and parameters.

At first, it didn't work, so I thought it was because multiple instances were necessary. But turns out that I needed an extra "world" frame that I didn't have, adding it without transforms was sufficient.