Hi Boxer,
I try to answer your questions below:
Q1- I have ISG-2000 with IDP blade. I have one core SSG-550 firewall with three zones trust, dmz and untrust. I want to use firewall of existing SSG-550 and IDP blade of ISG. How can i use only IDP blade of ISG not its firewall?
Depending on your network diagram, you may want to replace the SSG with the ISG-IDP or place the ISG-IDP to protect a network and leave the SSG in the core.
I would personally replace the SSG with the ISG-IDP so that you can apply the IDP inspection in the core of your network.
In this way you can keep the SSG as a backup device (if you don't have a cluster already).
Q2- I want to protect trust zone and dmz hosts of SSG-550. Where can i place ISG-Blade? May i need two IDP each for trust and dmz?
If you replace the SSG with the ISG-IDP you can control all the zone's traffic from the core.
Q3- What is the minimum configuration i have to do in IDP blade for IDP to work? I mean initially only IDP security policies configuration is enough?
Remember that to manage the IDP you need an NSM server.
From NSM, you can start configuring a policy for IDP and select as Attack the "Recommended Attacks" with Action "Recommended Action", and logging everything.
Hope this helps
Daniele