Depends on how much you have left over it may have been designed that way for price.
THis comment doesnt make any sense at all. All the energy and space on a ship should be utilized - youve already paid for it.
Put it into the special or weapon as Jessie suggested.
You need 10 fleets of ships, you can certainly start with 3 or 4 and successfully do missions - but ultimately you must have 10 mission ships. When you launch a new lead ensure its cost lies between 25% to 50% more than your previous most powerul ship.
With a properly structured fleet and well designed ships you can do any mission thats less than 3 times the fleet power of your leading ship - as your a beginner, Id suggest dont tackle missions or encounters with more than double the fleet power of your lead.
If you often lose ships following those instructions then your ship design or fleet set up is very poor and needs to be adjusted.
Theres always a remote chance of losing a ship to a rare type of enemy spawn - high defence swarmers or tinymen can both cause unexpected losses of ships. Its better to budget for the losses and attack the higher encounters rather than play ultra conservatively.