The name "cow and calf" was, as you say, applied to sets of switch engines which were originally semi-permanently coupled together by drawbars (instead of couplers) and which contained cab and booster units. I might add that both ALCO and Baldwin also built "A" and "B" unit switch engines in this same configuration as well; Oliver Iron Mining was a well-known user of such engines.
"Booster" units are locomotives without cabs. This term seems to have originated from the early products of EMC/GM wherein the "B" units did not have equivalent horsepower to the controlling, or "A" units. Some of the early Pullman-Standard and Budd passenger trainsets had, for example, a 1200 HP lead unit and a trailing 900 HP cabless booster. Later, of course, but not too much later, the cab and booster units became equivalent in power as we know them.
"Slugs," as I've always known them, are units with traction motors but without prime movers, receiving traction current from either one or two locomotives at a time. This is distinctly different from brake trailers, or brake sleds, which are simply cars (sometimes gutted and topped locomotives) attached to switch engines (or units being employed as switch engines, I should say) which are used to add braking power for quicker stops when switching. Because a brake sled is operated like a locomotive as far as the brake system is concerned (tied to independent, or locomotive brake) it adds braking power, even when air isn't used on the cars being switched. These also help avoid flat spotting of locomotive wheels due to locking up the brakes.
The ALCO-GE and Baldwin manuals here refer to road freight and road passenger locomotives as "A" and "B" units. However, the terms "cab" and "booster" seem to have been used very often by the railroads/railroaders at the time.
Finally, I always thought it humorous that the three unit C&O cow/calf/calf switch engine sets were referred to as "herds." Never was sure if that was railroad or railfan generated naming, but funny nonetheless!