Hello Shradha,
I see where you coming from , We have same setup as u talking about.
To answer your question, you don't need SMD agent or Host Agent for Monitoring your IDOC and
QRFC if you using BPMon as ur source of Monitoring, since BPmon uses the Plug in (ST-PI and ST-A/PI)to collect the information Via Read RFC.
So yes you can select the Cross Application or Interface Monitoring component as your Monitoring Objects with in Business Process Steps and Configure your setup for IDOC and QRFC.
Hope the information is helpful, if you want any more information please let me know.