Loaches aren't my area of "expertise", so to speak, but I know the basics and a 75 is going to be too small for one clown loach, let alone more. As Sprinkle said, they can regularly reach 12" or even more in an aquarium, and 10" is pretty much a given. They are also active fish and do better in groups. Unless you plan on acquiring a 6 foot tank sooner than later, I'd consider re-homing the loach.
With a 75 I'd seriously look at a larger school of cories, as "the more the merrier" philosophy definitely applies to cories, and you have a large footprint and plenty of volume. Six may get lost in the decor and plants and hardly be seen, and without the loaches you would have plenty of room on the tank floor for them to root around in.
I'm assuming the harlequin rasboras are a "must have" for the tank. Tetras I've kept successfully with angels and the rasboras include pristellas (x-ray tetras), head-and-taillight tetras, rummy-nose tetras, gold tetras, january tetras, dawn tetras, and glowlight tetras right off the top of my head, as well as cherry barbs. I know this list is not comprehensive; there are plenty of other species out there. In all cases, you'll want to be sure that the tetras are large enough or grown out enough that the adult centerpiece fish (assuming angels here) can't eat them. With angels, you'll want to avoid almost all barbs, a lot of tetras (serpae tetras come to mind immediately), and almost all danios. These fish are notorious fin nippers, and can tear an angelfish's fins ragged and keep the angel constantly stressed. I know people who have kept fish in the preceding list with angels, in large schools so that the barb/tetra/danio ignored the angels, but I also know just as many people who tried that strategy and failed. It's hit and miss and I'd just completely avoid barbs, danios and fin-nipping tetras to avoid heartache. If you decide on a different centerpiece fish, then the composition of your schooling fish may have to change to reflect the size and aggresssion of the new species.
WYite