Conjectural ships in SFB were put in for people to use in player campaigns. Therefore, I see no reason not to use them as Dogmatrix has suggested. I do think though that some "realistic" limitations need to be used.
If the Feds built a BB they would have needed to allocate the resources. A shipyard that built DN hulls would have to be used. For the time that it took to build the BB (5yrs? for the first one anyway) that shipyard would have been tied up and not building the other hulls (DNs or CVAs). Maybe they would have built the BB parts and pulled a DNG out of service to refit it (not likely unless one was so badly damaged that it would have taken as long to repair it)? This probably could have saved them a year or so, but the DNG would be out of action from that time on.
If the Roms get a KCR, that hull had to come from the Klingon's BCH builds. You'd have to allocate the Romulan facilities to do the conversion and limit other ship production accordingly, during that period.
If you wanted to build more than one KHK then you'd have to curtail the production of other firehawks. Possibly even delay a DN because R torps don't grow on trees.