If i developed several independent game worlds and character using Unreal, each with its own logic and rules. Can i combine those worlds and have characters traveling through different worlds?
You can use the “Level Streaming” system of the unreal engine. This is especially designed for many (sub)levels. This is also great for bigger teams, so everyone can work on their own level or part without disturbing the rest of the team, because in the end every “world” or level will become part of the big persistent level and can be turned on or off if needed.
In the unreal engine you can control either pawns or characters and you can give them any logic or “rules” you would like inside of the blueprint system or in c++ if you know how to code. As far as I know you can only possess one character at a time, but you can switch between those characters.
If you are asking because you would like to have multiple players for a multiplayer experience: Yes you can also have multiple different characters running around. You only have to spawn and possess them correctly so it all works fine 
Thank you so much for the quick response. I got a follow up question. What if the worlds and character was created by different teams that is not in association with our team, we do have the source code of these worlds and characters. Can we still combine these worlds and characters if they were all created by unreal? What if these were all created by different engines, like by unreal and unity, can we still use unreal to combine all these?
Your team should use separate software to create the 3d models. Why? Because then you can export them into some file like FBX or something which can be used by Unreal and other software. So yeah, you can use characters from other teams if you have those compatible data files (I don’t know what kind of different file types there are, but you can do it). This is also true for Rigs and Animation.
According to “worlds”… Well I guess that depends what you mean with “worlds”. If this is just a huuuge bundle of 3d models, then yes, you can do that on the same way as explained, but this may not be the best method, because the time to import all those files will be huuuge as well I would guess. But smaller chunks, no problem.
If you have functionality in your world, then you may run into some problems. Unreal uses the Blueprint system and C++ as the programming language. If I’m informed correctly then Unity uses also some visual scripting, but the programming language is C#. So If you create something in one engine and want to migrate it to an other engine you will probably not be able to. You have to export the files you use in the engine, so you can have a data type which is compatible with the other engine, like said file type “FBX” for 3d models.
I don’t know what you can export and what not, because I don’t use unreal for everything and the engine is not designed to do everything, but it can do pretty ■■■■■■■ much. You can create your own 3d models with brushes and create static meshes which you can use, but exporting them and use them in other projects? Could get messy. So you should start with several programms which focus on creating 3d models, like Blender or Maya, etc. So you can be sure to use those 3d models everywhere if your file type fits your needs