ES and FF each take up 14 crits, not structure. There are 47 Critical spots on a battlemech record sheet, not removing hands, lower, or upper arm actuators.
The best advice I can give is this.
Ferro-Fiberous armor benifits light mechs and light mediums.
Endo Steel helps Mediums and heavies, and light assaults.
I don't think anyone here said that you can't get both, I do believe that the general consensus is that FF Armor is almost patently useless, unless you merely want to sink cash into it, save for on extremely small mechs that only need 4-5 crit slots for weaponry. Even a Raven can mount larger Weapons, as can the Cicada, and if you rig it right, a Jenner doesn't even have the slots to carry both Endosteel and FF, merely lacking the critspaces to remain effective.
So, while yes you can get both, there really is no reason to, unless you -really- need those 9-11 extra armor points that you can get for swapping to FF on a light mech.
Also: I've not played in a couple days, I've been sick... they released the Flea? Since when? If not your Flea reference is absurd.
Edited by Discordantone, 15 November 2012 - 03:53 AM.