Hello GameMaker02,
What you have essentially done is regressed your workflow. If you combine your meshes for export to UE4, then your uv’s will have to be spaced accordingly. If you combine your meshes then your UV maps are also combined into one. If you have your uv’s occupying the same space, then they will be overlapping.
You do not have to combine your meshes in Maya in order to export/import properly. You can create your meshes in Maya separately and export them into a folder. It really doesn’t matter where you export them but this will make life easier. Then, when you select import in UE4, select all of the .fbx files you wish to import and then import them. More .fbx will take longer and such so if you have a good number of meshes then you can try importing a handful at a time.
Typically, what happens is a level is blocked out inside of an engine and/or modeling program. Once the layout is established with dummy/box/whatever meshes then the level is populated with actual assets.
This is a really nice tutorial to get you going on “blocking out” levels in UE4.