A lot of str8 users are drawn to Coticules for some reason, especially new guys. Maybe the "one stone fits all myth" I don't know. I don't hear too many vets or honemeisters talking about Coticules much. I too was taken in by Coticules back when I started with str8's, so much so, I became a vendor for Ardennes. Coticules are a fine stone, but if I tried to hone razors for customers using just a Coticule, I'd be in serious trouble. Personally, I use the Chosera 1k for bevels, followed by the Shapton GS Series stones, and then some type of finisher. Lately, it's been the Suehiro Gokumyo 20k. All the stones I use are fast, efficient and leave a smooth, keen shaving edge. I never use a Coticule to finish any of my personal razors. The edge just isn't there for me, if that makes sense, I know I can do a lot better.
As mentioned, no two Coti's are the same. They really are a finisher/touch up hone and would be fine for maintaining a shave ready blade, like Barber's did back in the day. Nothing is going to change the fact that they're around 8k, 10k is pushing it. Not just my opinion, but Ardennes as well.
Using slurry increases cutting action. This works pretty much with any stone. People use Naguras on Jnats. It gives a finisher more cutting action. This works with all Naturals. When you create slurry on a Coticule and start honing, you want to look for darkening. This indicates steel removal. Some are fast, some are dead slow. When you see this, you know your making progress. There's also the scratch test. You scratch the surface of your Coticule, if you see a distinct scratch, it's soft. If you don't see anything, it's very hard. This test makes more sense when you have several Coticules to view. It's usually accepted that Harder Coti's are better finisher's, while softer one's tend to be faster cutters.
I know it's a bit late, but I was going to recommend a synthetic, since you have a Zulu coming your way. The learning curve with Coticules can take longer than other stones, especially if your just learning how to hone, and even more so if your trying to do it on a small bout. It can be very, very frustrating, so be patient. Some have better luck than others, and this may have something to do with the stone they receive. I've been down that road.
Good luck to you.