Based on how you want to proceed your taking your project backwards and ultimately going to use a lot more memory space.
Spriter plugin system handles animation for you. You don't need to export to sprite sheet, unless you specifically need spritesheets. That's also very unlikely for the purpose of animation. So let's go over the post.
"My goal is to have an ouptut similar to a sheet like the one for teh main character in the jungle platformer sprite pack."
Do you actually need a sprite sheet? So far I only know that you need animation. What exactly do you need the spritesheet for? Spritesheets use a lot more memory and storage space than a Spriter object.
"So I put all the pieces of the monster together, use the bones to get the overall structure and movement done, "
Not entirely. This will only achieve a non moving object. This is the first step to using Spriter.
" create my keyframes, then export the keyframes of the entire creatures movement (as opposed to sheets for each component of the creature) frames.
This is not how spriter works. Keyframes are positions and angles for 1 part of your monsters part. ie it's one position of the leg at one point in time. The next key frame could be the last position in time in the animation. A keyframe is not an animation frame. It's point A and B locations at differnt points in time. The animation will move the objects position and angle from A to B during the animation process.
A(0time)- - - - - - - - B(10seconds later)
X - - - - - - - - - - - X
The above has Keyframe A and B. B is set to 10 seconds later and 200 pixels away from keyframe A. Let's ignore angles.
When the engine animates in game or tool the X sprite. It's going to create 600 "frames" or animation positions. You as the monster designer do not have to actually animation anything between time keyframe's A and B.
That's the point of keyframes and this form of animation.
"As long as Spriter can do that for me, then I am all set when I buy it."
Lucid says it's coming, and if you need it. Then sounds good :) Spriter makes animation easier even if you want to use spritesheets.
"Other wise I can just screenshot each keyframe, and put those screen shots together into a spritesheet I guess"
nope. Keyframs are not the same as a sprite sheet animation frame. They represent a part of the objects in a different place and time in a animation.
You can if you want animate and take as screen shot, but your overcomplicating your game design.
My suggestion is to try using spriter and watch the tutorials. Watch how many keyframes it takes to make an animation and how it all works. It seems by your posting that you might not know how key frames work in animation. All we want is for you to make the most informed decision.
But if you actually need individual frames. It will work and good luck :)