A topology only works on the feature classes in the feature dataset it is in. It is basically a database function that works in the database it is in.
Yes, you can set up the topology to work on just one feature class in the feature dataset. For example, you can say you want there to be no gaps and no overlaps between the features in the feature class.
My understanding is that if you want to include more than one feature class in the topology, they MUST to be in the same dataset as the topology.
A fast way to get all the feature classes in one dataset is to use ArcCatalog. Right click on the feature dataset, choose import, choose Feature Class (multiple). See image:
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