Yes, through Apple Health: any wearable whose app writes its data to Apple Health can feed Rhythm, because Rhythm reads from Apple Health, not from the wearable.
Rhythm reads from Apple Health, not from any wearable directly, so what shows up depends on what you have shared with Health. The Apple Watch and Apple Health page has the full list of what Rhythm reads and what it writes back.
What reaches Rhythm depends on what your wearable's own app chooses to write. Garmin Connect, for example, can write your workouts, heart rate and sleep to Apple Health once you turn that on — but a score like Body Battery is Garmin's own calculation, not an Apple Health data type, so it never comes through, however you set the toggles.
Open your wearable's app, find its Apple Health setting (sometimes called “Health sharing”), and turn on the data types you want it to share. Then, in Rhythm, connect Apple Health the same way you would for an Apple Watch.
There is no direct connection between Rhythm and Garmin, or any other wearable brand — Apple Health is always the bridge. None of this is required: your logged sets always decide what Rhythm suggests; a connected wearable only changes how confident a suggestion is.