Most 1st, 2nd, or 3rd Generation Variable Fighter designs intended for space use do have some type of FAST Pack to supplement their internal fuel tanks and extend their operating time outside of a planetary atmosphere. The need for FAST Packs is for much the same reason described in my previous post. Regardless of their airframe sizes, all of them used that same initial generation of thermonuclear reaction turbine engine technology that had some serious efficiency problems in space because they were using reaction plasma as a propellant. The few models that didn't require a FAST Pack, like the 2nd Generation VF-4 and 3rd Generation VF-14, were mainly larger frames that either incorporated secondary engines to reduce the burden on the main engines or simply were so much larger that their internal fuel storage was adequate for long-duration space operations.
Advancements in Gravity and Inertia Control (GIC) technology and heat exchange theory made in the 2020s and the early 2030s led to a new generation of thermonuclear reaction engine technology that was adopted by the last few 3rd Generation Variable Fighters (the VF-16 and VF-17) and became a standard feature of 4th Generation VFs. The new engine concept, thermonuclear reaction burst turbine engines, were a massive improvement in efficiency that more or less eliminated the need for traditional FAST Packs when paired with an adequately large airframe. With a vastly more efficient engine, they no longer needed high-capacity external fuel tanks to maintain enough fuel that they could burn it at accelerated rates to produce plasma as propellant. In the process, the whole FAST Pack idea underwent a radical reversal.
The first three generations of Variable Fighters used FAST Packs that were designed around those large conformal fuel tanks and supplemental rocket engines that were necessary to extend the VF's endurance in space. Weapons systems were added almost as an afterthought, to make up for the increase in size and weight. The 4th Gen, and later, VFs went with a new concept where the FAST Packs were focused on adding large amounts of weaponry and just enough additional fuel or boosters to offset the degradation in performance caused by the extra mass of that additional weaponry.
The first few implementations were pretty minimal, just small conformal packs with a small amount of fuel slush and maybe an extra pallet of micro-missiles like the YF-19's FAST Packs. There were some limited-use packs that were used by the 3.5th Gen VF-17 late type and VF-19 2nd production type, but it wasn't until the 5th Generation VFs that the concept really came into its own with HUNDREDS of additional missiles and other weapons.