I don't think individual pieces should be tags. I'd rather that be an explicit database table. But I do think "usual equipment" and "FIDE+compounds" and similar classes of piece sets could be useful as tags.
Yes, tags can't do some things that the database table is intended for, such as keep track of the names used for the same pieces in different games. Instead of using tags for individual pieces, it may make sense to use tags for common sets of pieces with tags like Pieces:Chess, Pieces:Shogi, Pieces:Xiangqi, etc. My issue with Chess+Compounds or FIDE+Compounds is that there are different sets of compounds. Fusion Chess and several related games also include royal compounds. To distinguish these, we might use Pieces:Chess+RBN-Compounds for games with the pieces of Capablanca's Chess and Pieces:Chess+KRBN-Compounds for games with the pieces of Fusion Chess.
Yes, tags can't do some things that the database table is intended for, such as keep track of the names used for the same pieces in different games. Instead of using tags for individual pieces, it may make sense to use tags for common sets of pieces with tags like Pieces:Chess, Pieces:Shogi, Pieces:Xiangqi, etc. My issue with Chess+Compounds or FIDE+Compounds is that there are different sets of compounds. Fusion Chess and several related games also include royal compounds. To distinguish these, we might use Pieces:Chess+RBN-Compounds for games with the pieces of Capablanca's Chess and Pieces:Chess+KRBN-Compounds for games with the pieces of Fusion Chess.