What is the ideal division structure for minors?
There isn't one. Because, well, "it depends"...
Mainly on:
1) What's your bottleneck?
IC, manpower, TC, oil?
2) What's your aim?
Does it really need to be Africa? Usually most provinces come with low IC, manpower and with some exceptions even low resources, added the long traveltimes and attrition Victor mentioned already above. The whole of Namibia gives you...
3) Whom or what are you fighting in what terrain?
Adding to Victor's hints:
Especially for minors, passively upgrading your units is your friend (passively=without allocating IC to the upgrade slider, takes quite some time but if you start early you can get marines, mountian units etc. more or less for free, their upkeep is cheap but the cost to reinforce losses due to attrition higher due to their higher base price). Same way you can get yourself some MOT or tanks. The usual upgrade line is: militia>cav>MOT/tank or militia>inf>marines/mtn. It's a bit gamey but IMHO absolutely ok (perhaps even a necessety) when playing minors.
Without knowing your answeres to the 3 questions above... my usual approach when I, for whatever reasons, go for Africa:
# Mtn/eng in transports to hop along the coast with amphibious landings (marines would be better, especially in jungles but if you can't afford the tech team Mtn/eng is the next best)
# garrisions to guard important spots (mainly regions with resource depots supplying your units or naval bases which your ships use as bases)... they are cheap, ok combat values and especially in jungle regions with attrition also very cheap to reinforce (additionally garrisons use no fuel which halves the attrition)
# MOT/ac to move around
# a few TAC (as low as 2 or whatever you have available) to either support my few MOT or even to kill off the lone enemy inf or militia with "ground attack" (takes long but often enough faster and more sufficient than trying to encircle such a lone unit)... having a few air bases in the production line to place them where needed is helpful and rather cheap
# alternatively to annihilation with TACs, in Africa especially with MOT overrun is usually better than encirclement (overrun=defeat a unit, see where it is retreating to and having a unit there when it arrives which destroys the unit on arrival)
# trying to concentrate on regions with some kind of IC, resources or at least a naval base or air base and leaving the other ones (which only burden your TC).
Addon: The 3-unit-stack Victor mentions I usually apply as well... but not in Africa where it is mostly overkill. But other than that a very useful organisation as soon as you have to manage more units (if that is the case as a minor, well, depends...)