Long time Marik here, but:
I'd run two 6-tank Lights and two 5-tank Heavies per Platoon.
I always took that "platoon" (the 10-15 line) to be a typo/confusion with Company, given that 10-15 is a Company in the manual.
5-tank heavy lances is new... I do recall reading it somewhere but don't remember where. Possibly on a forum, haha.
I think what TT was saying is that for a Light Company, he runs 2 platoons of 6 Light Tanks.
For a Heavy Company he runs 2 platoons of 5-Heavy Tanks.
I assume the 10-15 variable # for a platoon is to allow for some flexible formations based on what any given unit has.
For Example: Possible Configurations using 4-6/Platoon & 2-3/Company per FM:FWL
Company-10A = 2x Platoon-5
Company-10B = Platoon-4 + Platoon-6
Company-11 = Platoon-5 + Platoon-6
Company-12A = 3x Platoon-4
Company-12B = 2x Platoon-6
Company-13 = Platoon-4 + Platoon-4 + Platoon-5
Company-14A = Platoon-4 + Platoon-5 + Platoon-5
Company-14B = Platoon-4 + Platoon-4 + Platoon-6
Company-15A = 3x Platoon-5
Company-15B = Platoon-4 + Platoon-5 + Platoon-6
Any of those could be for Heavy or Light companies in theory.
The key is that by the time you hit Regimental Level that the total #s between Light or Heavy Regiments vary a lot with Heavy Regiments only being 1/2 to 2/3 the size of a Light Regiment in total #s but easily equivalent in terms of total firepower.