Your understanding of the poolish is essentially correct. However, it is possible to make poolish with a prefermentation period of from 3 hours to 15 hours or more. Of course, the shorter duration poolish would use more yeast than a longer duration poolish.
With respect to your specific questions, here is my response, using your numbering system.
1) and 2). In the bread world, where poolish originated, it is common to use 20-80% of the formula water as the weight of the poolish, although some bakers will use poolish as a percent of the formula flour or even the total dough weight. The 20-80% water range is the range that Didier Rosata, formerly of the San Francisco Baking Institute, recommends. The more poolish you use, the more acids and other byproducts of fermentation you will get, which will contribute to crust color, flavor, aroma and texture of the finished crust, but at the expense of a more elastic gluten mesh (stronger dough) and a higher risk of sugar depletion (the natural fermentiscible sugars extracted from the flour by enzyme performance). This means that you may have to add some sugar or diastatic malt (about 0.5-1% of the total formula flour) as part of the final mix. Otherwise, a nice brown crust coloration in the finished crust may be lacking.
The amount of yeast to use in the poolish is as a percent of the poolish flour, not the total formula flour. Typical prefermentation/yeast profiles, also from Didier Rosata, are as follows:3 hours: 1.5% fresh yeast, as a percent of poolish flour
7-8 hours: 0.7% fresh yeast, as a percent of poolish flour
12-15 hours: 0.1% fresh yeast, as a percent of poolish flour
Note: Prefermentation assumes a room temperature of 80-85 degrees F, with water temperature = 60 degrees F
Either ADY or IDY can be used in lieu of fresh yeast. For ADY, use one-half the weight (or percent) of the fresh yeast; for IDY, use 40% of the weight (or percent) of the fresh yeast. If a different prefermentation temperature is used, or a different water temperature is used (e.g., to shorten or lengthen the prefermentation time), adjustment to yeast quantity will be required to achieve comparable results.
3) You simply combine the poolish with the remaining ingredients as part of the final mix. However, you want to wait for the break point (the point at which the top of the poolish collapses into itself and then recedes), or a short period thereafter, before conducting the final mix. If an autolyse is used (the classic autolyse), the poolish should be added after the autloyse rest period, to avoid incorporating any yeast into the dough and acidifying it during the autolyse rest period. Depending on the amount of poolish used and its temperature at the time of the final mix, it may be necessary to adjust the temperature of the remaining formula water to achieve the desired finished dough temperature (around 75-80 degrees F for a typical home application).
It usually takes a series of tests to determine the best amount of poolish and the best prefermentation time to use to achieve the desired end results.
For a recent example where I used the poolish method, see Reply 17 at http://www.pizzamaking.com/forum/index.php/topic,7225.msg76431.html#msg76431
Good luck, and please let us know how things turn out.