For me, I'm looking for a mattress that is 8" or less, due to the height of my head board. I have a solid cherry wood platform bed that I love. The second factor is cost. Often more is not better. Better is better. Therefore, if one layer gets the job done I can in most cases afford the highest quality mattress since I'm not paying for all those extra layers. That's my thinking at least, flawed as it may be :-)
If you're not fussy, and the thought of a way too-soft mattress doesn't bother you; then you could do that. I wouldn't recommend it. I bought two 3" ~ average 25 ILD. Way too soft for me even though I can still "feel" the wood-base, and it doesn't get any firmer if it's one 8" vs. 6".
Given your 8" requirements, and assuming you're both a back/side sleeper, and the size of an average American (huge assumption):
I'd probably recommend getting a 3" of "x-firm" (because it provides support and insulates you from the hard platform, unlike "soft")
Then probably 3" of "firm" (maybe "medium", maybe) as a transition.
Then, reserve 2" of comfort layer to your liking, only after trying the first 6". if you find the 6" is way too firm and more than enough support, buy a super soft (low ILD) 2" topper; otherwise you might decide a slightly-softer topper would be good. A possible risk is that you bottom out on the 2" soft topper into a too firm core. It's the toughest part of your dilemma.
8" is probably underkill, but I kind of like the idea of pushing the envelope on the least amount of latex needed. I'm thinking, if you're going with a thin core, you're going to need a very firm transition between the wood-base and the top layers.
This is just my intuitive gut feeling about it after my initial fiasco. I'm a noob, in the learning/experimenting process, so please take this with a grain of salt.