Hahahaha...you kill me with laughter, newbie in this game......you are one of the best when you play Dara, and always have been....hard to beat.
As for the original question, operating a single fleet is optimim, but as you will see now, or soon, most of the best players will have 2 to 3 fleet types, and even 4 in a few months as things really dry up, 2 is really not much penalty, as for 3 it is still marginal, but 5 and above is an absolute killer.
Go for the Beoing 377 too, as good as Dc6. Dont worry about fuel price for the next 15 years, its cheaper than watre and will remain so for a long time, fuel efficency is not a factor in this game for many many years..........as was in the real 1950s and 1960s where oil flooded the earth and was a penny on the dollar.
Goodluck and alway fly the hell out of a route.