A modular, scalable/expandable, MSE, rack assembly for physically supporting information technology, IT, gear of one or more information handling systems, IHSes, the IT gear having different dimensions of at least one of height, width, and depth, the rack assembly comprising: a rack chassis comprising a plurality of interconnected panels forming a volumetric space having a front section and a rear section, both with opposing side panels (212, 214) forming a front access space and a rear access space, respectively, between corresponding opposing side panels (212, 214); a plurality of guides located within interior surfaces of the opposing side panels (212, 214) at the front section, the plurality of guides running in a lateral direction along the front access space towards the rear section; one or more block chasses (125a-d) each having a frame that provides a block height, which enables insertion of at least one layer of N fully functional IT gears within the block chassis, the frame defining a corresponding information technology, IT, bay, having a total width that enables attachment to opposing guides on the opposing side panels (212, 214) and configured to support insertion of one or more IT gears having at least a first size dimension selectable from among multiple supported height, width, and depth dimensions ranging from a fractional or partial width, height and depth dimension to a full height, full width, and full depth dimension of IT gear insertable into the particular block chassis (125a-d), wherein the block chassis (125a-d) is physically inserted into the front section of the rack chassis and held in place by at least two opposing guides on the opposing side panels (212, 214); a power bay located within a segment of the rack that is specifically designed to receive one or more power bay chasses; a power bay chassis inserted within the power bay; a power and management module, PMM, slide-ably inserted into the power bay chassis and having a rear conductor plate, wherein the PMM comprises a plurality of power supply units, PSUs, that provide power to all functional components of the one or more IHSes; and a modular vertical busbar assembly electrically coupled to the rear conductor plate of the PMM and extendable in busbar segments along at least a length of a back of the front access space within which one or block chasses (125a-d) having IT gear are inserted. a rack chassis comprising a plurality of interconnected panels forming a volumetric space having a front section and a rear section, both with opposing side panels (212, 214) forming a front access space and a rear access space, respectively, between corresponding opposing side panels (212, 214); a plurality of guides located within interior surfaces of the opposing side panels (212, 214) at the front section, the plurality of guides running in a lateral direction along the front access space towards the rear section; one or more block chasses (125a-d) each having a frame that provides a block height, which enables insertion of at least one layer of N fully functional IT gears within the block chassis, the frame defining a corresponding information technology, IT, bay, having a total width that enables attachment to opposing guides on the opposing side panels (212, 214) and configured to support insertion of one or more IT gears having at least a first size dimension selectable from among multiple supported height, width, and depth dimensions ranging from a fractional or partial width, height and depth dimension to a full height, full width, and full depth dimension of IT gear insertable into the particular block chassis (125a-d), wherein the block chassis (125a-d) is physically inserted into the front section of the rack chassis and held in place by at least two opposing guides on the opposing side panels (212, 214); a power bay located within a segment of the rack that is specifically designed to receive one or more power bay chasses; a power bay chassis inserted within the power bay; a power and management module, PMM, slide-ably inserted into the power bay chassis and having a rear conductor plate, wherein the PMM comprises a plurality of power supply units, PSUs, that provide power to all functional components of the one or more IHSes; and a modular vertical busbar assembly electrically coupled to the rear conductor plate of the PMM and extendable in busbar segments along at least a length of a back of the front access space within which one or block chasses (125a-d) having IT gear are inserted.