What a lot of questions. Let me see how far I get.
Slug -- well, I suppose you're printing on a much larger sheet. You can treat everything up to the physical size of the sheets as 'slug', but 10-20 mm should be enough for anything you need to put there. Remember, it's outside the actually used area of your design.
2 copies on the same page -- in that case you cannot use ID's default crop marks and you will have to draw your own. See next:
Crop marks -- sure, you can draw anything you like, anywhere you want. As long as they're outside the actual design. Dov's remark applies to 'standard' documents (ads, pages for a book), but as long as you restrain yourself to the simple rules of crop marks (don't place them near art -- overlapping the bleed area and running into the slug area is good; use as thin lines as your output device can handle, but not less than 0.25 points; use [Registration] color, even if you don't intend to colour-separate, as you never know what will happen with the document in the future). For further safety, put them onto a separate layer, so you can switch them to invisible with a single push of a button in the Layers palette.
Take a look at ID's own crop mark options to see what you need: the actual crop marks themselves, perhaps registration marks as well (the circle things in the center of the slug area) to align multiple copies (easier to cut), perhaps color bars (to check the quality of the colors). Adding the job name and date (for archiving hard copies) never hurts.
The script you mentioned earlier adds crop marks to a selection of objects on the screen, but I've never needed it so can't vouch for its usefulness. Is the script in your Scripts Panel? If not, copy it to the correct location (for a Mac -- where?), select an object and double-click the script to see what it does.
As for printing, well, opinions may differ. Directly printing from ID should work just fine, but -- again -- for safety reasons, you might want to export to PDF, check everything with Acrobat, and print from there. Having the document as PDF ensures you can send the document directly to another printer.
(Phew what a long post.)