On Battlefleet Koronus is written that Laser-burners (p.35) can be installed in Dorsal or Keel Slots of Frigates and Raiders. What about the Transports?

Did this mean that normally, on Frigates and Raiders, the Lances can be mounted only on Prow Slots?

On hulls smaller than Light Cruiser, a Lance can only be mounted on a Prow slot.

That's right, Lances can only be mounted on a prow slot on anything =/<Frigate. Another thing to note is that in such cases, the firing radius of the prow-mounted lance is only 90 degrees, instead of 270. IE, may only fire in the front arc, not to port and starboard as well, as is the case with =/>Light Cruiser sized vessels.

That's not a special case, the Prow slot on Frigs/Raiders/Transport can only fire forward no matter what.