I've never read any comics from the Hellboy universe before. But i would like to, especially the B.P.R.D. titles. Recently while browsing the wiki page i came to know that there are a few B.P.R.D. omnibuses. Can i jump right into them? ... If i start reading them without reading the hellboy issues first, then will i miss out on anything?
... I think there was a run in the past in which Hellboy and the members of the B.P.R.D. appeared together in the same books ... Do i need to read those first before reading the B.P.R.D. omnibuses?

What about the Abe Sapien omnibuses and Witchfinder omnibuses. Do i need to read any other series before jumping into Abe Sapien and Witchfinder titles?

Also are there any crossovers between these titles (Hellboy, Abe Sapien, Witchfinder, B.P.R.D. etc.) and do i need a reading order ... or can i just pick up any of these omnibuses and start reading them?

Thanks.