talon8 is absolutely right: the best way to go about this is to use a thermometer. However, it is still an interesting question how the expected cooking time varies with the weight of a roast.
It is such an interesting question that in 1961, SIAM Review published a scholarly article entitled "On Cooking a Roast". To be a bit more precise, the question that the authors answer is this: Suppose we have two roasts with exactly the same combination of identical tissues, and the same shape except that the one is a scaled (blown up) version of the other. Let's weigh both roasts and compute the ratio between the two weights; let's call that ratio r. Suppose furthermore that the roasts have the same starting temperature, and we will cook both until they have both reached some given (higher) temperature at their centre. What can we say about the times that both roasts need to cook for?
The answer is that under these assumptions, the bigger roast will need r^(2/3) times as long as the smaller one.
So, assuming each small roast (700 and 950 grams) has the same shape as the big one that the recipe writers used (let's say 1750 grams), for the 950 gram roast you'd expect to use (950 / 1750)^(2/3) = 0.665 or about two-thirds of the cooking time, and for the 700 gram one you'd expect to use (700 / 1750)^(2/3) = 0.543 or just over half of the cooking time. In particular, for the 950 gram roast, you should expect to roast it at 200C for 20 minutes, then turn down to 180 and roast for around 0.665 * 1750 / 450 * 12.5 = 32 minutes more. (1750 / 450 * 12.5 = 49 minutes is the average time they would recommend for the 1750 gram roast, and we're using that as the base for comparing our 950 gram roast against.) Then check with your thermometer!
Finally, I think for the resting time, you might as well use the original 30 minutes, because that's not (only) about getting the internal temperature to a certain level (although it does contribute to that - the heat will spread out through the meat), but also about things like the muscle fibers relaxing and the like, which are not covered by the article.
Absolutely! Metal skewers contribute to cooking the meat from the center as they pick up heat from the exposed parts and conduct it throughout.
It is a good idea not to crowd your metal skewers with pieces of food to promote this. Not crowding the skewers also exposes more surface area for the food to caramelize. Doing this on a wooden skewer runs the risk of burning the skewers and losing food into the grill.
Best Answer
There is no way for us to give you absolute cooking times. There are too many variables, among them:
You will note that the number of kabobs is not a factor.
Instead, you need to learn to check for when they are done to your liking. This will come from experience. Kabobs are generally too small to use a thermometer on; you would need a very high quality one like a Thermapen in order to get a reasonable reading.
All that said, it is likely that you want something on the order of 2-3 minutes per side, but there are far too many factors to make that a concrete recommendation.
You can start by cooking just one kabob, or even one chunk. When you think it looks good, try it. Of it is overcooked, do the next one for less time; undercooked, cook a little longer. After a couple, you will know about how long you will need for your particular set of circumstances.