Hi cocoa, welcome aboard!
I’d also recommend starting with a D whistle. It’s inexpensive, readily available, reasonably-sized for adult hands, with tons of instructional help and materials available. The basic blowing and fingering techniques can also be applied to the Native American Flute and the recorder.
And as I’ll describe below, a whistle can easily be converted temporarily to a pentatonic instrument if you’d like to try that.
I’m holding a Susato Pentatonic Recorder in my hands now. Mine is the model in the key of F, and the lowest note (bell note) is C. I’ll explain this discrepancy in a moment.
The Susato pentatonic recorders are listed on this page here:
http://www.susato.com/susatorecorders.html
First, it’s not really a recorder, in the sense that most people here would understand. Rather, it’s more like a Susato pennywhistle with several of the holes absent (and the B1 hole enlarged) in order to play a pentatonic scale. Like a whistle, there’s no right hand low pinkie hole, and there’s no left hand thumb hole. It’s really a simple system diatonic instrument, not a Baroque-fingered recorder.
I think the reason for the recorder designation is that the target audience are music educators who might normally start their students on the recorder as a first instrument, but who subscribe to the pedagogical theory that pentatonic instruments are a better choice for preschool beginners. The Susato is a good solution for that application.
Second, the Susato recorder is described as a plagal instrument. This means that the designated “key” of the instrument is the note located in the middle of the scale - what most whistle players would think of as the “G fingering” note. In order to compare the Susato pentatonic recorder to its equivalent whistle, you need to look at the lowest note, given on the Susato web page. In the case of my F instrument, the lowest note is C. And, in fact, the instrument is the exact same size and length as a C whistle.
Similarly, the pentatonic sopranino C that you mentioned has a bottom note of G. Which means that it is identical in size to a high G whistle. And as most people will tell you, these are very tiny. To the point that many adults can barely play them even by scrunching their fingers together. The pitch is also quite high, and can be rather piercing.
So if you do get a Susato pentatonic, I’d recommend nothing smaller than the soprano “G” instrument, with the bottom note D. This is the equivalent of a D whistle, and should be easily playable.
If you do choose a standard D whistle instead, it’s easily converted to pentatonic by covering several of the holes with a small piece of tape. Masking tape works well for experimenting. Simply cut two small tape squares and cover the #1 and #5 holes (i.e. T1 and B2) like this:
[mouthpiece]@oo o@o[end/foot] @ = piece of tape
Voilà, you now have a plagal pentatonic G whistle like the Susato. The G note will probably be a bit flat, especially in the 2nd octave, but you can blow a little harder to push it back into pitch.
For a slightly different pentatonic scale, cover the #1 and #4 holes (T1 and B1) with tape:
[mouthpiece]@oo @oo[end/foot]
This gives you an authentic pentatonic scale starting on the low D note (all fingers down).
As for choosing a D whistle: If you’re interested in NAF, you might like an inexpensive Clarke original, Clarke Sweetone, or Shaw whistle, all of which have a breathy sort of sound.
Hope that helps, and good luck with your musical adventure.