[Alneyan]

Actually using the Master Computer *will* gain space for Cruisers and above, as if you have Master Computers, you don't need to put Bridges/Life Supports/Crew Quarters. So even with Computers I (which gives a 40kt Master Computer), you will gain 10kt on your Battle Cruisers. However, if you are afraid of Computer Viruses, you may put one Bridge/Life Support/Crew Quarter plus the Computer so that you will not lose all movement when the Computer goes down.

Bases mounts do increase range, as Satellites mounts (if anyone bothers to research Large Satellites) and Weapon platforms mounts, so it probably explains why you were outranged. Were they using their heavy direct fire weapon which does a lot of damage but only fires every four turns? If they didn't have much PDC on their weapon platforms, you might consider using fighters to take down the weapon platforms, but if there are few settlers on the planet, capturing the planet without destroying it will prove to be hard.

The AIs may have declared on you only because of colonization rivalry. If memory serves right, the AI claims as its own any colonized system and all neighbouring systems, so every planet of yours in these claimed systems will bother me. It poses quite a problem with Partnership as well, since the AI will know where are your colonies and may want a lot of these. Or of course you may very well be the Mega Evil Empire, if you have a nice lead.