@drjb was right. My indicator bulbs arrived today and this picture shows what the 3-bulb fixture looks like with the indicator bulb installed.
Hue (the supplier of this smart bulb) calls light output from it Warm White. I’m not sure the name of the bulbs in my ceiling fan, but they look much cooler in comparison. I wasn’t happy with this appearance so I removed the indicator bulb from the ceiling fan and put back the original bulb.
If I were to purchase two regular LED bulbs in Warm White, then I could replace all the bulbs in my fixture. The result might be an acceptable color match, but drjb’s advice was fresh in my mind. Therefore, I located this new smart bulb in a single-bulb fixture that I use a lot. My indicator ratio will only be 1.00 for this bulb, which is a little disappointing. I can always come back to the idea of matching bulbs in the ceiling fan at some later date if I want more mileage from my smart bulb.
The basement lights are already a mixture of florescent-style bulbs plus a single bulb, so color difference there is irrelevant. At this location the bulb will function as an indicator to monitor usage, just as expected.
I turned on the wall switch and watched in Sense for my new smart bulb. It takes about 5 seconds for the bubble to appear. Turning the switch off, the bubble takes a full minute to disappear, although the integration reports status as N/A within 20 seconds. This feels clunky compared to Kasa smart plug integrations or native Sense devices. For perspective, however, many room lights are used for either minutes or hours at a time, so measurement delays in seconds are not numerically significant.
This post from last year confirms that Hue is not part of the 20-device limit. It says the Hue bridge can address at most 50 bulbs.