This might be veering a tad off topic, apologies in advance if so.
In general, a "balanced" warship would devote about 30% of its mass to defense, offense, and power, with the remaining 10% for sensors, accommodations, etc. Each of those categories can have a fairly "loose" definition, defense can be armour, shield, PD, AMM, and Stealth for example. Likewise making "specialists" like Battlecruisers (Pocket Battleships) with Battleship guns with Cruiser defense, or Sensor/Scout ships like an AWACS aircraft can be done as well.
Now since in the game, accommodation is more or less a set "ratio" of mass taken, it can really bite into the 10%, unless it doesn't as one can count it in the other category as well. Which leads to the interesting quandary that speed can be a type of defense as well, both in regard to a base hit chance as well as the ability for one's PD to be more effective. So making a "Fast BB, CA, CV, or even DD" can be done by combining the power and defense aspects without much effect on the overall offensive output.
On the tanker issue, the problem is that realistically one would be required to support/guard the logistical supply line, else a smart enemy would simply sever the chain, and play an attrition game. So by increasing operational range by the use of tankers, one reduces the offensive weight of any action, since some portion must be used to escort the tankers. Only if one has reasonable "space superiority" can one "risk" not escorting them. (static defense JP's which are not stabilized for example)
Overall, I find myself using even engines on civilian commercial vehicles, and odd engines on military, with only Fighters having a single engine, the loss of optimal output is outweighed by the redundancy of reliability, same thing goes for reactors, RP-wise I find having a port and starboard or fore and aft reactior, each being able to handle the load of the entire ship, "right" for a warship.
In closing, thanks to nuclearslurpee for putting on a maths treatise. Could have jumped to the final answer of 42 though...