SPAD 13 had about 150 horsepower, but that was considered a lot at the time. Same with F units, in fact to replace a 4-6-2 Pacific steam loco required 2 F units, for a 4-8-4 Northern they needed 3. The advantage was lower operating and maintenance costs, plus the fact that they could be controlled by a single engineer in the lead loco. The switcher and "road switcher" design was primarily for that problem, the F units had very limited visibility out of the cab when trying to do switching (shunting). As for design rules for any kind of machinery it depends on what era you're talking about - workers were considered expendable 100 years ago, if one got killed on the job you hired a new one, and nobody gave a thought to toxic waste or pollution. Modern times require environmental protection standards and safety standards for all machines, so design regulations have been in a constant state of change for the last 50 years. many new laws have a "grandfather clause" where previously existing safety hazards are allowed to continue for a period of time depending on how dangerous they were before the new law, and how practical it would be to update to the new standard - for example if the loco is in the shop for a rebuild anyway the law might require it gets updated at the same time.