1. You really only need one good positioned scanner for a good defence. This is because when two scanners detect activity, only the higher level one activates.
2. Use your code gates well. Preferably in range of your scanner. Code gate's filter makes it slower to put access programs on it
3. Keep your scanner 3 connection points away from the closest net connection. This makes sures that the scanner is activated as soon as the stealth begins
4. Upgrade your scanner and code gates. Best do this early game when you are less exposed to stealth
5. Start using stealth attacks. More experience in attacking gives you more experience in defence. Learning what works and what doesn't work is important. Once you do this, you can start sharing experience on the forums

With your questiom about passives...
I don't think so, but thats just based of my stealth attacks. I'll look closely next time I use it.
Upgrades for gold?
Scanner (the earlier, the better) > gates > sentries (cheap) > black ice > turret > guardian (it's not that great early game. Shields are only like 80-100 extra firewall)
I usually upgrade things every two levels. 1 > 2 > 4 > 6 etc because they change looks each time
