1) No such rule exists, so yes you can have an entirely infiltrating army. However note that an
IC without Infiltrate has no way to infiltrate alongside a unit they want to join (as the
IC must be deployed on the table before the infiltrating unit can be), so that may mess you up. Fine to join in reserves and Outflank though
2) erm not explicitly that I'm aware of, but its fairly easy
(4/6+(1/6*4/6)) is your BS4 example. FOr BS3 simply replace 4/6 with 3/6. Probabilty of just hitting added to the probabilty of getting a 1 on the first roll and subsequently hitting.
No supplements, howevr the *excellent* Horus Heresy books from FOrgeworld give you all of that. You do NOT need the huge tomes to simply play the armies, although they are absolutely stunning - all you need are the
Red Books which provide rules for, from memory, 10
SM armies, mechanicum as well, and includes
DG, Emperors children and your primarchs.