One problem I forsee with rails and PAC builds is fighting anything with a fighter bay. Those little things would be amazingly difficult to punch out, especially if the enemy cmdr hops into one. You would need to put huge pressure on their main ship or be an exceptional shot to survive that one. To that end once I get a better feel for rails and plasmas, I might diversify into frag cannons. They've a big enough spread to probably take those fighters out, maybe. Then again I doubt a DBS has any business blowing up the big three in the first place.
Rails are the single biggest threat to SLFs in the game.
Unless you can keep the SLF very close, you'll be easy pickings for anyone that can time a rail shot. A dirty drive DBS is fast enough to open a gap with an SLF, even if it misses the opportunity to snipe it as it's launched. That micro-gimbal/snaplocking of fixed weapons makes it easier than one might suspect to land rail hits on small targets.
Fragment cannon, on the other hand, are virtually useless against SLFs. The spread is extreme and even at close range it's rare for more than one or two of the pellets to land. You have to be caught within a few tens of meters of the weapon. I've occasionally been able to drive off CMDRs in all frag or even frag/PA FAS, FGS, and Clippers with nothing but a single Condor. A DBS might have enough of an agility edge to be more of a threat with frags, but it much harder to stay that close than it is to open up a gap.