I think this site may help you

http://archive.ics.uci.edu/ml/datasets.html

What do you mean by multi-database dataset? Do you mean a dataset distributed and inter-connected into various databases? Need for further explanation.

i suggest you find a single dataset, and convert some portion of it into another type of data format, say you've got a big dataset in csv and you turn one third of it into xml, i presume here that you want to integrate these multi-data formats into one database.

You can see UCI Machine Learning : 

http://archive.ics.uci.edu/ml/datasets.html