I think different Power Armor models are really not that much of a problem, if you consider that they are somewhat like walking tanks in that sense. It is not uncommon that a prolonged war will see many different models of certain units, with smaller or bigger improvements, or just like you say, simplifications. The best examples to look at, are the tanks in WW2 and the high number of different models, particularly with medium tanks.
Well, but most tanks in WWII were progressing or taking different characteristics as the war went on in order to respond to the advances in the enemy's own tanks and changing roles.
Here we don't really see any reason for the T-60 as of yet.
T-51b was an improvement upon the T-45d, it gave the user better protection, better mobility, its power supply lasted much longer and so on. We don't really know of any flaws in the design that could be improved upon. It could very well be the best the pre-war industry could come up with at the time.
T-60 on the other hand looks inferior to T-51 in every possible way save for one, possibly. The jetpack. It could be explained, that T-51 was incompatible with a jetpack, so they had to design a new armor that could utilize it, but at the same time forsake some of the protection, power supply and mobility.
