1. If you're not going to use Starforge to do the whole map, you should use it last. If you have sprites or other things that SE will remove, you can use the SCMIE tool (
http://www.staredit.net/index.php?download=363) to save them in an external file and load them back in later.
2. There are several ways, the most common and straightforward being the randomize action for switches. Details here:
http://www.staredit.net/index.php?tutorial=61
3. Any map from 20x12 to 256x256 will work, with one caveat. If the map has non-standard dimensions (i.e. anything you can't make in SE), it will crash if it's the first map you play on b.net.