another shadowhawk?
I kid, I kid.
let's see, you've got a medium weight lance and they're mixed with one assault. lots of energy weapons and direct fire capability, mostly energy based. you've also got;
50
55
80.
hm.
let's see what else...
6/9
5/8
5/8
generally, if you're using the Crab as your scout/vanguard, another 5/8 'mech makes the most sense, something in the 60-75 ton range. if you go slower, you need to make it a support platform (Bomardier maybe-an LRM boat with the same movement profile, or an Archer, for 10KPH less cruise, that you can park and use to support your lancemates)
if you go lighter, then you have a problem; your line straggles.
while most Shads do a pretty good job of being 'light fire support' designs, it's not going to grant the range or firepower output to really be an asset to the rest of your lance.
so I'd go with something like a Catapult, Archer, or Bombardier (the last because this is an SLDF lance, or used to be, anyway) where you've got a bit of medium weight to go with the Flashman's all-energy icebox slugger configuration and the Crab's all energy light/medium point man config.
something with good dispersal volume that can pinch hit as terrain control in an SLDF setting, (aka thunder w/Fascam as an option, or indirect fire as an option).
yeah, I'm liking an Archer as your fourth man. a bit slower, but not so slow it can't keep up entirely, good plating, decent secondary weapons and a powerful support configuration that's almost moronically easy to use.