Thanks @Nicholas_Bokulich for providing all the great details!
@hemprichbennett - you might be able to randomly select sequences with this tip, but it might be easier to do externally from QIIME. Alternatively, there might be a few RESCRIPt tricks you could use if you have a list of IDs you want to play with first (and filter from the larger dataset)… something like qiime rescript filter-taxa - see here (section on ** Creating a classifier from RefSeqs data**).