Late January Update:
We went out of state for a few weeks over the holidays, and when I returned the alkalinity was very low (below 5 dKH) and the Chaeto wasn't doing so hot despite NO3's nearing 50ppm. They probably got a little more food than they should have while we were gone! So a couple water changes later (so easy with this size tank) and the alk is closer to 8 and the nitrates are ~12ppm. The Chaeto seems happier and has sped up growth again.
On another note, this is the first time in months that the NO2 reads a true 0! I knew the fish weren't going to be affected by low levels, but it is nice to know the tank is getting closer to reaching some equilibrium.
So I want to give it a few more weeks to make sure the parameters are stable, and then I will proceed with adding the first bubble tip. The pair of clowns don't seem as cozy anymore, as the larger one tends to keep the other on one side of the tank. She doesn't bite or do anything physically abusive, but it does not look like they are really pairing up yet. Crossing my fingers that the smaller one hadn't already morphed to a female state prior to being introduced to the other. Also, the larger one is clearly growing at a faster rate (expected when changing sexes), and I'm bit worried as to whether she is going to outgrow the tank sooner than I expected.
I picked up a Tunze 3155 ATO before Christmas, and after seeing its size (in tank) decided that this isn't the way to go with such a small tank. Part of the joy of a nano is keeping it simple (not too much equipment to maintain or go south), and the visual real estate this ATO would take over just didn't justify my lazy desire to stop putting a small cup of DI water in the tank daily.

. I might look at smaller units, but I already have more equipment on this tank than I would really want.