The Ozuku Asagi is a good choice for straight razors because it is a hard and fine natural. Ideally you would use a piece of Ozuku Asagi to rub against the larger stone, referred to as a tomonagura.
The uchigumori is used for polishing the sides of a knife or sword to enhance contrast and bring out the beauty of the steel. The Uchigumori Mark has is a soft Uchigumori - ideal for this polishing use but not really something to use as a nagura stone. Classically at least. Good uchigumori is becoming increasingly rare so if you have an interest in aesthetic sharpening, get it while you can. You could always experiment
If you need some Ozuku Asagi tomonagura, just send me a PM. Highly recommended for this usage to get more out of your Ozuku Asagi. Now you can use a true nagura in conjunction with stones like the Ozuku Asagi, like Koma Nagura or Botan Nagura (which I have), but this results in a coarser slurry that using an Ozuku Asagi tomonagura.
For straight razors, do consider the finer CBN and polycrystalline diamond. Compounds finer than 1/4 micron are all the rage on the straight razor forums these days, going as fine as 0.025 microns - 10x finer than the 1/4 micron products. A particularly nice combination past 1/4 micron is 0.1 micron CBN followed by 0.025 micron Polycrystalline diamond - both on nanocloth or kangaroo.