4th Sword of Light Brigade with the 10th, 11th and 12th Sword of Light Regiments?
This is logical and just shows that House Kurita knows the value of concentrating their best units in bigger units for a bigger impact on the battlefield when 'Mechs are more numerous.
I think we would have the 1st, 2nd, 3rd and 4th Sword of Light Brigades.
The question is more should we consider them pure 'Mech units or mixed units?
There's probably a 5th SoL Brigade, which would get us to the 15th SoL regiment mentioned several times in TROs 3025 & 3039. If during the Succession Wars these brigades were reduced to the strength of regiments, and then reorganized into the regimental structure we know since 3025 (as prior SoL organization descriptions report that the regiments were reorganized several times) then this actually goes a long way to fixing what seemed to be prior errors. And there's a long history of units amalgamating as times become hard; in BT, look at the Fusiliers of Oriente description in the 3025-era
House Marik sourcebook; in real life, look up the British Army, which has undergone repeated series of unit amalgamations since WWII.
As for the 1st Galedon Brigade, things don't have to make nice orderly numbers. While the army of the Republic of Georgia has its 3rd Brigade consisting of the 31st, 32nd, and 33rd Battalions, the US 1st Marine Division consists of the 1st, 5th, and 7th Marine (Infantry) Regiments, as well as regiments numbered for 4th Marines, though 4th Marine Regiment (the headquarters) is part of 3rd Marine Division; the 2nd Marine Division has the 2nd, 6th, 8th Marine Regiments (and the headquarters), but also the three battalions numbered for 9th Marines, though no 9th Marine Regiment headquarters exists. 3rd Marine Division, meanwhile, consists of the 3rd Marine Regiment's battalions and headquarters, as well as the headquarters of 4th Marine Regiment.
There's no rhyme or reason to this, many military formations evolve haphazardly, and the 1st Galedon Brigade may well be the same way. The SoL Brigades, however, as the apex of DCMS prestige units, are more likely to remain per the strict structure.