Is it assumed that everyone with the relevant skills for the ritual knows these?

Or do you need so find some specific grimoire?

I have used that players must find the specific grimoire for Rituals. Might have given them leeway for summoning lesser & bestial daemons.

Agreed.

"Access to the forbidden library of XYZ" or "The aid of the Cabal of whateveryournamewas" are the sort of thing I would expect a sorceror to make infamy-based acquisition tests for, at least for the first time he attempts it.

Some Lesser rituals are easy to find - certainly I'd let you off, as said, most or all that effort for summoning a single horror, but if you want to summon a herald of Tzeench or even a Feathered Lord, you're going to be needing the details on a specific ritual. Such beings demand exacting precision on their invitations, or they tend to arrive... tetchy .

Depends on the ritual. Some are basic and well-known by many, others are more obscure/rare. There's A LOT of rituals out there.