[Suicide Junkie]

I have 1800kT colony ships available from the start, but I don't think that includes the ability to build a 1700kT propulsion system for a 100kT command module, and have it be linearly efficient compared to a more reasonable design.

Of course it dosen't... after putting on the colony module, and all the required cargo bays, how much space do you have left?
40%? ... 20%? How much will you use for engines, supply tanks, defenses?

Most basically, if a ship design has enough of a speed advantage over its enemies, it can do silly things in combat compared to weapon ranges. This can cause imbalances with unrealistic tactics that take advantage of the lack of opportunity fire in the combat engine, such as ramming or hit-and-run without the enemy being able to fire back at all.

The thing to note is that, yes, speed should be an advantage! In tactical, it may be abused vs AI, but not vs humans. In strategic its AI vs AI, and thus perfectly valid, IMO. The second thing to note, is that in order to get a really good speed, you will have to strip out almost all of the armor/shields and guns...

Sure, you can fly circles around the enemy, but you can't do much damage, and your ships will drop like flies as soon as the enemy gets a chance to fire on you.
PS: Don't expect super-speedy rammers to be very effective, since you'll have no armor to bulk up your impact damage, and you just spent thousands of radioactives to build those precious engines.

In any case; I played many games like this at home with two other people... Each liked their own tactics, and are solidly convinced theirs is best.
(Me = big on defenses - if you can't kill me, I can't lose)
(Brother = Big on Weapons - boom, you die, I win)
(Father = Big on engines/speed - weak attack, and no defense, but if you can't hit, it don't die.)