You're correct in that the D5P and D6P are Fast Patrol Ship (Pseudo Fighter) PF tenders.
I read up in SFB Volume.III. which lists the ships as D5F and D6F, which is also the rules volume where the K-RKL is introduced, along with the K-MON, AD5, AD6, D5V, D6V and D7V. Actually 50% of the D5, D6 and D7 variants appeared in this volume back in 1988, plus the "K" refit.
However, they are an allowed construction or conversion in SFB camapigns and F&E games.
I, myself, find both a bit of a liability in games, mainly for the amount of incoming fore they attract once an enemy knows what they are and what they can do.
If I wanted to sulk around and wait ages for Plasmas to charge up, I'd play as Romulans.
However, there are odd occasions where an initial heavy blow is welcome.
I'd suggest that anyone curioius about the conversion simply make a duplicate of a D5 or D6, label it D5F or D6F, replace the Disrupters with either twin or quad PlaF launchers FA and try them out for themselves.
It might seem like good idea against AI opponnents but against human players, not so great.