Yes you can use the 140 grit flattening stone. I have the 150 and have not tried the others, but from following this forum for about 1-1/2 years now, I think any of those three stones will make you happy. Of course all stones are slightly different, but it is very likely that at that grit range the differences are subtle.
I like my 150, but I also would recommend another stone before going to a 1K (like the 400 Latte, a great stone) if you don't have one.
What's your goal here? Is something that low necessary? I'm just trying to get a better understanding of your need/requirement. I've had great success thinning and re-profiling carbon steel with my Gesshin 400, so you might not need something so extreme. Do you intend to remove a lot of material often? Although it isn't as low a grit as the ones you've mentioned, the Nubatama 220 is a nice rock as well.
The 120 bamboo is only for single bevel knives, if used for anything else it will dish extremely fast (its a fast wearing stone regardless) and the mud it produces will often scratch the side of your blade.
The 150 bamboo is a very slow wearing stone and has a good cutting speed. It is not extremely fast unless a good deal of mud is built on the surface and tends to leave a finer scratch pattern than the grit rating would suggest.
The 180 bamboo is probably the best for feel and consistent mud production. It's not the fastest stone but will produce a fine silky mud and defined grind lines better than some of the other stones along with a consistent drag in the sharpening.
The 220 pink bamboo I'll add some comments on because its one of my favorite stones. It's kind of a mix between the 120 and 150 in wear rate and cutting action. It's faster than the 150 wears slower than the 120 but produces a fair amount of that sandy mud that can cause minor scratching issues if sharpening to a low angle. It's another one I typically only use for single bevel traditional knives but can be used as a all around stone if careful.
With all of these stones the numbers mean very little IMO, the 220 pink cuts deeper scratches than all but the 120 bamboo, the 180 is the finest, and the 150 is probably the most well rounded for a wide range of sharpening. They all remove steel very well but can be task specific depending on the type of knives you sharpen.
Why the need for a coarse stone?
A coarse stone is the base from which a sharp edge is created. Sure a 400 grit stone can work but its slow and simply cannot perform the tasks a true coarse stone can handle with proper efficiency. Grinding with too fine of a stone causes inconsistency and simply takes too long, try and grind a chipped yanagi with a 400 stone and watch the hours stack up.
@Jason B. - I agree with your explanation above for the need of course stones. I was just posing the question to the OP to understand his situation. If his knives are in good shape and well maintained, something sub-4/500 may not be necessary, but if an overhaul's waiting in queue, then a brawler is absolutely recommended. Your breakdown of the grits is also very useful in determining the right stone. Now get some videos posted