Hi Ads,
Have not really had time recently to pop in and read what has been happening on this forum, but just dropped in now to see what people have been up to and saw your request on the mammal guide.
I'm not sure just how keen you are on mammals, but let me just punt a Capetonian's perspective here quickly and say that there is more to South Africa's mammals than just the Kruger National Park. Whilst the Kruger might have lots of the big and easy to see mammals, many, if not most, of SA's endemic or near-endemic mammals probably don't even occur in the park. The mammals that you are most likely to encounter in the Kruger are common throughout most of Africa and you can see even more impressive numbers of those same mammals in East Africa (although they are still incredibly satisfying to watch when in Kruger!!).
If you are a little like me in that you are super-keen on mammals (I am just as crazy about mammals, reptiles and amphibians as I am about birds), then I would think you would not be doing terribly well on a 3 week trip in SA covering areas right from the Cape to the Kruger and only coming away with a list of 20-30 species. I would think that you should expect nothing less than 40 species and, with a bit of luck and some work on your side, you should be able to crack 50 species. This would not be unusual on a 3 week trip right across the country.
As to the guides, Smithers is probably the best but, as the name suggests, only covers the land mammals (here is the Capetonian in me coming out again!). The cetaceans are not covered and, of course, we have an absolutely stunning near-endemic here on the west coast in the form of Heaviside's Dolphin, surely one of the best mammals that this country has to offer! Given the fact that most of the coast around the Cape is currently thick with Southern Right Whales at the moment as well, best you brush up on how to id those too so that you can pick them out between the many Humpback and Bryde's Whales that are also around at the moment.
Haven't really gone far back enough in the forum to see your full itinerary, but saw that you mentioned Sani Pass somewhere. If you are going up there, keep a look out at the top for Slogget's Ice Rats too - they are fairly common up there and are one of the few endemic rodents that you have a very good chance of seeing.
Good luck and I hope you have a fantastic trip with lots of spectacular mammals!
Trevor