I don't think there is any "lowest" altitude that can be assigned to a jet. It really has more to do with efficiency. A jet is much more efficient at high altitude than near sea-level, so generally speaking, the higher the better. ATC usually tries to accommodate this by allowing jets to climb to altitude as soon as possible, and by not requiring them to decend far out and truck in at a low altitude.
For short flights the cruise altitude will be lower as a necessity of being a short flight.
Anyway, that's my $.02.
The climb and maintain 3000 is not specific to those aircraft. We get it all the time with heavies in the military. Its a function of where you are taking off from (south of a busy class B airspace) in our case. So, we usually tool around at 3000 doing 249 waiting for that climb.
It all matters on what the aircraft needs to do, or go. Also, fuel efficiency is another thing. Here in New York, American frequently ferries their aircraft from Kennedy airport (JFK) to La Guardia airport (LGA). They're only 9 nautical miles apart. The "cruise" altitude they fly is 3000 feet. So, there really isnt any "minimum cruise altitude" for any aircraft. The only low cruise altitude is the ground...That's as straight-forward as it can be put.
Also, let's define "maximum service ceiling." That is the altitude at which the specified aircraft is unable to climb at over 100 ft/minute (read this somewhere a while back). It all depends on aircraft performance to determine service ceiling height. Hope this helps.