Reading through Battlefleet Koronus, I noticed that while it specifically lists plasma drives for Grand Cruisers and Battlecruisers - it doesn't list a single warp engine for those ship classes.

What is a poor Rogue Trader to do?

( well, not so poor if he has a Battlecruiser or Grand Cruiser hull although he may be poor after properly outfitting it )

Both Grand Cruisers and Battle Cruisers have a note saying they can use "Cruisers Only" components. I take the sane approach that this means anything that can go into a Cruiser can also go into one of those vessels.

Calligos Winterscale's vessel from Edge of the Abyss is an Avenger Grand Cruiser that has Strelov 2 Warp Engines, which gives my theory a bit of credit.

I will agree with the provision that from reading the GC and BC entries, that at least in some cases, it sounds like there were ( or should have been ) one or two more selections as far as warp engines are concerned - at least one GC class appears to have had a warp engine unique to that class ( and gave that class a bad reputation ).