The way Permanent Summons work now, the Dirge would not be able to summon skeletons because of the ones spawned bay barrows. It has been at least implied however that the in 0.34 Kael may be copying the way Xienwolf chnaged it to work in Fall Further, so there is a one to one correspondence between the summoner ans summon. That would let the Dirge summon a single skeleton, which still isn't that much. You could give it twincast too if you like.
If I were to change it to a normal summoner, I'd let it learn Death II too, and make its summons start with amphibious, boarding, and/or even water walking.
It might be better just to give it a separate, unit-specific spell.