Alternatives of BitThief to Download Torrent Files Sequentially
I highly recommend Transmission (https://www.transmissionbt.com/). Best BitTorrent client on Mac. 
There is no sequential download option, but you don't want that, it is not only damaging to the BitTorrent network, but downloading sequentially also should result in slower download speed for you in general. 

If it is about streaming the videos you download, aka start watching while downloading, give Popcorn Time a try. It is a streaming client based on the BT protocol and works nicely.