1. What are all interfaces developed so far relate to sap-xi?

The Interface developed by SAP-XI is Inbound Interface and
Outbound Interface.

2. How system does if I don't want w/tax for some of vendors for some period & again I want to use it?

If you don't want w/tax for some of the vendors for some
time, you can change the settings in FK02 in withholding
tax tab and deselect the W/H tax assigned for the vendores.
When it is again needed, again select the tab or assign the
another W/H tax to the vendors

3. What are the impotent things when we are configuration of mm module?

we are using in purcahse orders integreated to fi modules