I think an easy solution would be to alter the network in a way so that the uploader could check a box or an option that says "Don't uplaod files larger than x MBs without the download having y sources". So therefore, when someone decides to just download a 600MB file from just one source, thus monopolizing this one source, the uploader could force the downloader to have perhaps 3 sources before downloading the file. More sources will decrease the amount of time any one source is monopolized.

Such an option should definitely have a max so it would not be abused.

Or the above option, the two tiered system would also work well.