As is the case for Anselmo López, there are players doing segundeos with the middle finger. This requires a lot of training but gives the advantage to enable segundeos not only on the adjacent string but also on all the other strings below. For example after playing the third you have options to do the segundeo on the second and the first. In this following video the first four segundeos are done with the index and the rest with the middle. There are players doing all these only with the middle.
Segundos with the Middle Finger
