Your ships are actually darn fast, given you are using ion engines. Personally, I would sacrifice quite some speed for more armor/weapons/magazines.
My own doctrin asks for 1 engine per 1.000t or 25% of mass used for engines. Your ships have dedicated between 37 and 45% to engines. Now, being able to close with the enemy or evade the enemy is all good and well, personally, I prefere to crush them with superior firepower
Another thing is the quite different speeds, your ships can reach. If they are supposed to work together, having similar speeds makes sense. If you want to put your DEs between your main group and the enemy by detatching them, they should be a bit faster but not by a whole lot.
There is actually a command to do that.
F12 screen, 2nd tab (Special Orders/Organistation). Select the ship you want to detatch, hit "Escort". The ship will form its own TG. Now, on the left, there is the "Protect Threat Axis" menue. Here you can set how far out and to which direction the escort will try to stay from the main fleet. If you want a DE to be 100.000km forward and somewhat to the right, you select "Distance:" 100.000 km and "Offset:" to 20° clockwise.
Be sure to hit "Save escorts" once you are done. After you have finished with all escorts, hit "Recall escorts." Now all your escorts will join the main fleet again. Whenever you need the escorts detatched again, just hit "Deploy Escorts" an all the escorts you saved, will automaticaly deploy and try to get into their assigned position.
Note: Fleet training is vitaly important for the escorts to perform well or they will be scattered all over the place.
Magazine space seems a bit low.
Less than 7 salvos for your DDG is unlikely to be enough. My own rule of thumb: 15 to 20 salvos are a target, 10 salvos an absolute minimum.
Your DE is a bit better, but belive me, you will go through your AMMs stock like there is no tomorrow. Raising mag-space to 35+ salvos might be a good idea.
The tanker looks fine, well armored, but what the heck. Personally, I allways put a ton of maintenance supplies on my tankers, so they double as supply ships.
To make a collier, just put a crapload of magazines on the ship. Be aware, however, that magazines are considered military components, so the collier will be a military ship and require maintenance.
Make sure to tick the "Tanker" "Supply Ship" and "Collier" checkboxes, so they can actualle reload/refuel your combat ships.
As Erik says, your DL is the biggest ship with the biggest active sensor, making it _THE_ missile magnet of your fleet. upping the armor might not be such a bad idea
