How can I preview (watch online) videos while downloading the torrent? How can I preview (watch online) videos downloaded from torrent? 
I've tried Torrent Stream Controller with Ace Stream Engine for Android but it seems to be only for torrent tv and I even couldn't force it to work on my device.
Sequential download feature is more common in current torrent downloading apps, e.g. you may find it in Flud and tTorrent apps.

It can be a bit problematic to find video player which supports playback of unfinished video files in various formats. VLC isn't perfect, but quite usable for this.