in mtg terms, a red and blue card is actually both so its affected by whatever affect only one part of it.
The imp can cast lvl 1-2 fire attack spells. Devil's trident, as a whole, is a lvl 2 spell, that is both fire AND dark, and its still an attack spell. From my point of view, since its both, it still count as being fire, so, castable.
Same for harpy. She's arcane and air, but she is still affected by whatever affect arcane.
So until an official answer happens, i'd say its better to make it work until we figure out its unbalanced, then change it after.