Keleth says:
Hm... I guess space is cheap, we can let it be unlimited, and if it becomes a problem, solve it then.
An elegant solution :) I think that works well.
As Harrigan indicated, I too VERY often use NPCs without sheets. I keep NPC stats and stuff separate, so on my GM sheet I'll just have lists of 'NPC avatar + name'. It's really great for style and helping the players quickly identify with a visual who is talking in dialogue.
Keleth - if it is helpful to see an example in action,
HERE is a post where I used multiple NPC avatars from the GM sheet for a back-and-forth dialogue including multiple NPCs in a single post.
And
HERE is an example where a set up multiple avatars for use on a PC, as the character would change physically depending on how he channeled his abilities (toward the bottom of the sheet).