I'm far from an expert. What I know is what I've experienced, heard and read. I don't make starters usually because dry yeast is too easy. Plus my beers usually are about 1.045-50. Not too much for a 11g pack of hydrated yeast to handle.
That being said, your questions is are excellent and should be addressed by the experts.
The question about temp and growth is interesting. Worry more about temp first then pitch rate? For outstanding results what my experience has taught me is that pitching an appropriate population of healthy actively fermenting
yeast produces the best beer. Hands down.
One vial of yeast? How about a two gallon batch and then top-crop and pitch into your five or another 2 gallons. I think the idea is that you don't want much yeast growth but healthy activity at the right temp.
For #4 Lagers or ales. I think at ale temps under-pitching will still net you your attenuation. However, under-pitching a lager will give you problems at a cool temp according to John Palmer. A tidbit from the member's only content...