The Ethereal Player’s Guide suggests that PC Etherials should be built on 6 Forces, I think that they should match the Force count of other PCs.
6 or 7 Forces is a good level for an all-Ethereal game, or to keep up with a party of Soldiers and Sorcerers. It is clearly inadequate to keep pace with a party of full fledged angels or demons. However, according to In Nomine, once an Ethereal reaches 9 Forces it has the potential to be a god.
The question then becomes:
“what is an acceptable god for a player character?”
Named deities from well known pantheons seem like a singularly poor choice, as their loss would significantly impact a game world (certainly more than one relatively unknown angel). Besides, if a spirit is that well known, it seems odd that it would be on the bottom of the god scale.
On the other hand, if you allow the full range of spirits, you call into question one of the core principles of the setting: Celestials have overpowered and displaced Ethereals.
In my mind the best type of ethereal spirit to mix with a party of Angels and Demons would be a minor tutelary deities, a metropolitan genius loci, or some other small god frequently thought of in the collective. This certainly includes such things as Alvar, Muses, Nymphs, Iye, Sidhe, Valkyires, but could also include sports mascots, local legends, or mythic civic patrons. Of course, just because a piece of the local psyscape has as many Forces as a god doesn’t mean it is one. Worship as a deity is still necessary for that last step.
Gods and Pop Culture
Part of why I think the Ethereal Player’s Guide advocates for weaker Ethereals is that many of the lesser gods have become adulterated over time. Most of us think of fairies and nymphs as tricksters and eye candy, but not gods. And I would certainly suggest allowing more pop-culture driven representations to exist on the lower end of the power scale.
However, if a fairy, elf, nymph or similar has been stated out as a god so that it can keep up with Angels and Demons, the player really should be their best to keep to the mythic source material. Nymphs aren’t pretty sex objects, they are gods of rivers and springs and groves. Someone playing a 9 Force Nymph should give serious thought to what natural feature they sprung from, and also why they emerged as an Ancient Greek deity rather than some other (more indigenous) spirit.