For rifles, and I would expect all pistols except maybe .25 Auto, the cutting of powder "logs" has no impact on the accuracy of the load.
From my understanding, every shot from every gun will discharge some unburned powder. Considering EXACT weight as being the path to accuracy has never worked for me. In fact, consistently throwing charges by volume tends in my rifles and handguns to produce slightly more accurate and consistently accurate ammo (smaller mean and std. dev.).
Ball powders tend to burn hotter than stick powders.
However, there is no way to choose one over the other until you have tried several in you gun to see what works best.
For a 40 gn charge weight, controlling the charge to +/- 0.4gn is a good as it gets--and is much better than even the premium factory ammo is held to.
One "problem" is that most rifles are never fired enough to determine what is "best" and many that are will be "shot out" before the testing is over. However, do not even think that just one or two 3- or 5-shot groups tells you what the load will really do. If you see more than a 0.5" change in group size for a 0.2gn change in charge weight, you are seeing much more of the randomness of the result than any truth about the mean accuracy of the charge.
1) What would be the best type of powder to use, ball or rod? For 9's, Mak's, .380, 40's and 45's and also for 308 for both the 550 UCS and the Saiga?
There is no way any one can tell you what your guns will prefer. For light pistol target loads, I prefer 231/HP38 (spherical), AA2 (spherical), and Solo 1000 (single-base flake that burns cooler then most). For magnum loads, all my magnums prefer 2400 (flake) over 296/H110 (spherical?). My criteria is always accuracy and not velocity.
In my .30-30s, IMR 3031 (logs) is about as good as it gets. In my .30-06s, IMR 4895 (logs) is about as good as it gets. One propellant my AR likes with many bullets and weights is IMR 4064 (log). If I was loading 7.62x39, one of the first powders I would try would be IMR 4198 (log)
2) any reason why 308 NATO (hotter round then the 308 Winchester and not recommend apparently for civilian sporting rifles?) would be a bad idea to use with the 550 or the Saiga?
That is a matter for the manufacturer to determine. I can not imagine a need to push the .308 over SAAMI limits.
3) And last how many rounds approximately (give or take a 100 or so) will a pound of power last for?
It is all dependent on the charge weight. There are 7000 grains to a pound. 1 grain = 0.000142857143 pounds. For a "typical" pistol load of 5.0gn, 5.0gn equals 0.000714285. So 1 lb will load 1400 rounds. A 20gn charge weight is 0.00285714 lbs, so 1lb will produce 350 rounds. So a 40gn charge weight will produce 175 rounds.