There is always a chance of it happening, but I've found when Serpaes are kept in an appropriate shoal and a decent sized tank they keep the nipping to themselves. In the end though, it's not worth the risk since there's plenty of other tetra's that would make better companions for the Angels.
i have 5 serpais recently bought the label on the tank in petshop said community never said fin nippers as it did with barbs and the likes, i have the in a fairly large tank along with the following
1 loach,
3 black widows,
3 glass cats,
4 danios, 2 zebra & 2 leopard (1 is long finned)
i havent noticed any problems with fin nipping the danios and the widows can hold their own if the serpai annoy em
i had a few serpaes with some cories in a 20 gallon, they nipped everything including me
i moved them to my 180 and they calmed down though, so i think it really depends on how much room you give them