Why is it impossible to use these two together? According to the OP, both only take a single crit in the CT and each Leg. The CT and Legs each have 2 free crit spaces available. The arms and side torsos certainly have enough crit spaces to spare 4 crit spaces each for both Endo and FF.
Probably because I was mentally filling in a gap: the OP's allocation only uses 11 slots, where 14 are expected.
I was think that this was when it was first introduced. Most of the items below were not even planned out by the design team in the 80's when FF and ES were first introduced.
I have it as TRO 2750 when they were introduced. Was there a book before that with Star League equipment?
Nah, most of what I'm talking about are in the current core books, TechManual and Tactical Operations, and are technologies introduced in the late 3050s and early 3060s - and the Clans started in TRO 3050.
So, reconsidering this as a historical what-if... what if FASA had created endo and ferro with those aspects instead of what we know, and only considering Star League and Succession Wars tech.
1. The effect of endo-ferro-XL, the common weight-saving spam move:
Arms: 4-6 cs. Side Torsos: 5. Head: 1. This gives you 19-23 critical spaces, before adjusting for sub-250 engines. It's not such a big problem for lights, since they have a hard time using the space; but they can't really use DHS with the combination. Gauss is impossible.
Mediums could run into problems, especially the 4/6 troopers which have crit-slotted engine heat sinks already.
2. Even dropping ferro:
Arms: 6-8 cs. ST: 7 CT, legs, and head: 1
Slightly better, though splitting the AC/20 is still mandatory, and even the Arrow IV Artillery is barely possible. You would need to be quite sparing on the DHS. At least you can mount Gauss now.
3. Dropping the XLE instead, because to heck with XLEs:
Arms: 4-6 ST: 8 Head: 1
It *looks* like a lot of space, but Arrow IV is impossible again (not that it'd be all that easy without the weight savings of the XLE anyway...) while the AC/20 is still mandatory split-location. Gauss is still barely doable in the torso, but not the arm.
I'll fix my assessment thusly: why do you want to nerf
mediums? ;D