CPLT-C1 Catapult - Twin LRM-20s with limited ammo and effectively no defense? Not ideal, in my opinion, AND it's a Heavy.
CPLT-C4 Catapult - Twin LRM-15s make this rather effective at LRM spam while the limited ammo lasts, and then it's a Heavy in-fighter to be feared. I'd gladly put one or two in a HEAVY Fire Lance, but not a Medium.
DV-6M Dervish - Reasonable firepower, good mobility, and adequate armor. Nothing spectacular, but adequate all around. There are better choices, but it would do.
LCT-1M Locust - This unit is excellent for raiding and harassing, in the right hands, but totally under-gunned for a Fire Support role except possibly for a fast Light formation. It's likely to survive a battle due to its speed, but that won't save your front line. Pass, quickly.
TBT-5J Trebuchet - Adequate LRM capability and good maneuverability make this a viable choice for a "cavalry" type force, but the jump capability takes away too much from its intended role compared to the -5N model. Depending on the force I'm using it with, it's either a reasonable choice or a mediocre one.
TBT-5N Trebuchet - Twin LRM-15s provide excellent LRM spam while the limited ammo lasts, but the shallow bins are a "feature" rather than a flaw, because the thin torso side armor won't last more than a couple of hits. I've seen far too many of these 'Mechs blow up when the unpadded ammo bins take a hit. You MUST use up the ammo before losing the side torso armor, but then it turns into a semi-decent brawler with 3 MLs and reasonable 5/8 mobility. It's not perfect, but probably my #2 choice for the role.
VLK-QA Valkyrie - The single LRM-10 rack just isn't adequate for the tonnage, and the single backup weapon offers little defense. The 5/8/5 speed isn't quite enough to protect it against the fast units that would typically be used to attack a fire support unit, and it can't out-fight them. Quite simply, there is no reason to take this 'Mech unless it's all you have.
WTH-1 Whitworth - The WTH delivers the same twin LRM-10 barrage as a DV and packs similar short range firepower, as well as similar armor, in a package that's 15 tons lighter and costs far less. The difference is primarily in speed: 4/6/4 instead of 5/8/5, which is fine for supporting a Medium front line of HBK, CN, ENF, and VND units, but a bit slow for backing up a more mobile formation of 5/8/x movers. If cost or tonnage are a constraint, then the WTH offers the best "bang for the buck", but it's not the best unit out there overall. Once the ammo bins run dry, the triple MLs come into play, and it's generally more than adequate as an in-fighter against the already hammered units remaining on the front line. With no tonnage or cost constraints to make it preferable, it's simply less 'Mech than the heavier options, but will hold its own surprisingly well against most of them.
HBK-4J Hunchback - This thing is remarkable. It delivers the same LRM firepower as a DV or WTH, and then converts into a powerful brawler once the ammo runs out. It shares the depressingly thin rear side armor of its HBK siblings, but that ceases to be a problem when the bins are empty. It also shares their decent heat sinking, as well as their decent armor, and packs 5 MLs, making it nearly as fearsome in close as a -4P. It makes the perfect bodyguard for a Fire Lance, and is my FIRST choice overall, but I wouldn't want a full lance of them because I'd rather have a couple of TBTs and GRFs providing the bulk of the ranged support, with one or two of this thing to protect them while contributing to the main mission.
BMB-10D Bombardier - Again, it's not a Medium, and is poorly protected. The right tool for some other job, not this one. Pass.
CRD-3R Crusader - The twin LRM-15s deliver effective long-range support, but the heat sinking is rather marginal, forcing occasional firing of only one rack, which leaves it only slightly more effective at sustained fire than with twin LRM-10s. The secondary weaponry is rather impressive on paper, until you realize how little you can fire in a sustained engagement. Then there's the issue of LRM ammo, SRM ammo, and MG ammo, and a tendency to go up in a blaze of something that I would not consider "glory". Worse, the SRMs cannot be fired into the side arcs, as they are leg mounted, leaving the 'Mech somewhat under-gunned against anything that gets around it and out of its front arc of fire. Basically, it's a dangerous opponent, but with major weaknesses which can be exploited. As with the CPLT and BMB, it's also a Heavy, so that's another strike against it in this role.
The GRF isn't mentioned, but it's quite capable of providing fire support, both direct and indirect. Perhaps not my first pick, but I'd probably add one to the lance.