Seems there are mixed feelings about the Aizu. I have four of them, and as I have said in reviews in the past, they are fairly inconsistent in terms of edge refinement from one stone to the next. The whiter stones seem to be different from the green ones and then those that have loads of brown/gold mixed with loads of renge(?) seem to be different still. However, none of mine have been in the 1k range that I can recall.
My softest is probably probably around HS 20 or so (unmarked) and it is chalky in use, cuts fast, leaves the bite from a 1k stone but the refinement of a 3k stone. I think that is here everyone gets hung up and why people try not to put grit ratings on naturals. That edge can be interpreted differently from person to person.
My next hardest (2) are almost identical and are both marked at HS24. These still feel chalky but harder yet cut faster with their loads of black renge(?). Finish on the edge on these is clearly more refined. Still a bite level like a lower grit synthetic, maybe the 1200-1500 rage but the edge feels to perform beyond that, maybe somewhere in the 3-4k range.
The hardest stone is different. It does not cut near as fast and is noticeably finer. Still loads of bite for its refinement level but probably somewhere closer to a 2k in terms of bite and a 4-5k in terms of edge refinement, feel.
Now, all of these seem to bite into my nail closer to mid grit 1-2k stone but you can feel the tactile difference through food of finer or coarser finishes. This is particularly noticeable when doing something like peeling potatoes on the board with a gyuto. You can feel one bite and resist the tough potato skin but melt through the flesh. However, the harder stone leaves an edge that kind of slips through the skin with little resistance but it is still there. In stark contrast to my Yaginoshima suita, which is probably closer to something in the 8-10k range. You get no tactile feedback of any kind when going through the potatoes skin, it is as if there is no skin to begin with. Like a hot knife through butter or slicing through air etc.
That is simply one example and my take on things. As for where to put the stone in a rotation as compared to an Ikarashi, for me that is hard. Why, because I find these stones to be fairly similar. Most of my Ikarashi have been harder yet coarser than my Aizu but not by much. Say my finest Aizu is 3-5k then the finest Ikarashi is 2-4k. But the thing is, practically speaking, the two finish so closely that I would not use both in a progression. Now, my softest Ikarashi was closer to the 1500-2000 mark but even that is about in line with my softest Aizu...I know, I know, we are not talking about running them together but I am trying to get my point across.
Simply put, I agree with Otto, as per usual. I would use it in the 3-5k range to replace a synthetic. Me, I don't normally mix synthetic and natural stones after 1k. I either switch to naturals or stick with synthetic stones. Or I opt to run all synthetic and switch to a natural to finish--sometimes I even step backward from a 6-8k to an Aizu or aoto. Anyway, I find my finer Naka-to to be a good replacement for something like a Rika 5k.
Natural stones, an addiction and a curse, I tell you.
Now I am going to sharpen, have fun.