First off, the small log mill is different from the mini mill. I'm assuming you are talking about the small log mill since that's the picture you posted.
I have one of those and I've used it for milling the floors and inner walls of my cabin. I used a husqvarna 450 with an 18" bar.
The mill has worked pretty good for me. Because of the single post design, I noticed its wants to wander a bit more than a full sized mill. Its light enough and not as bulky as using a full sized Alaskan mill. A huge plus, its quick to change cutting depths.
The instructions say its for a 20' bar max, but I have used my 32" sthil on rare occasions.
Strangely enough, Granburg uses a different system for mounting this to the saw than the regular mill. It has two bolts that tighten directly onto the bar. You want to keep a close eye on it when you first put it on the bar so you don't crush the bar rails.
It will put small dimples in your bar, but it won't affect anything.
As I said, the Small log mill worked well for me. Well enough that I bought the 36" version as well.
I do have the Mini Mill as well. I don't use that as much. I get far more accurate cuts from the small mill. Also that really throws sawdust up in the air. Besides the helmet/mask combo, be ready to wear a dust mask as well and try to get upwind. I also like a hooded sweatshirt to keep the chip from going down my neck.
Hope that helps.