hi Megan, those are great questions!
yeah, in general you have to generate sounds until you find one you are into and then you can adjust the pitch, etc
there isn't really a way to "paint" tones like in the tune too: my goal was to make something that didn't require a ton of fiddling to get interesting SFX so I followed the model of tools like bfxr. if there's a ton of interest maybe I could make an "advanced" mode, but I don't see it happening any time soon.
under the hood, the blip tool uses a set of parameters for defining sound effects (such as pitch, speed of pitch change, etc). technically, if you're really motivated (and willing to risk breaking your game) you can go into the bitsy game data and edit those parameters directly. it's not documented anywhere though so it would require a lot of experimentation. could be fun though!
for your other questions
- "random" sounds are as random as you can get within the parameters of the blip tool, yes
- I forget the difference between "greeting" and "meow" honestly - I think one tends to be higher pitched and/or shorter?? but I forget which
- length vs speed: length controls the total duration of the sound, while speed controls how fast the sound "moves" through its pitches (if that makes sense)
hope that helps explain things a bit - let me know if you have any other questions!