This may sound oversimplified, but there's no substitute for command points! This is usually the premier advantage of production races. They have more colonies with more starbases. When Tachyon Communications is chosen at the 250 RP level a formidable (missile) fleet can be raised early in the game and supported with little to zero CP deficit, though not always.
Use missiles, because beam ships without the battle scanner can only hit starbases or ground installations. You can build and bring beams, but you'll need the missileboat loaded to the hilt to hit elusive targets! Use fast armored ECCM nukes (chaff to absorb hits), followed by fully modified nukes then fully modified mercs. Fire the mercs second if they have Class III shields.
The first step to preparation is define the objective — know what to prepare for. Is this a limited campaign or is this a total conquer campaign?
Eliminate when possible! A limited campaign can whittle them down to size and allow you to build more starbases in conquered worlds to increase fleet size before the final assault. Although it's painful, extra ships can be scrapped if the BC crisis demands it once your goal is accomplished.
When the CP limit has to be busted, remember to heed the need for speed. That is, run the gauntlet as quickly as possible. Get a decent warp drive — Ion or better — to cut down on travel time. Also, try to build transports as close as possible to the action to cut down on travel time. Last but not least, build up the bank balance 200-300 at least, or better yet 500 or more, and have a planet or two for trade goods to survive the BC crisis. It's okay to go -50-100 for a little while when you're properly prepared financially. You'll use transports as you conquer; as the number decreases through attrition, build and send more as the budget allows. This allows a long-term military campaign that's survivable by limiting the shortage to -40-80 BC. Less by building trade goods.
As stated, when you conquer, you'll expend excess points as marines are killed. One CP for every four slain. Unless it's a low-pop outpost, you'll also need to reinforce the garrison to prevent the natives from becoming restless and overthrowing your hard-won gain, which results in an additional lowering of CP demand. Load up the conquered colony — losing it will extend the time you're in CP deficit. With trade goods production, you can shave your BC shortage to a bearable level, or possibly break even.
Try to take at least one high-population planet to feed people to low population planets.
A cute trick to employ is: assimilation by starvation! If the high-population planet is full, you can starve it a bit by going -1 or -2 food which results in 50-100k starvation to get it just below maximum. Example: the population is 15/15, and the actual pop is 15,031. After one round of starvation the pop will be 14/15. Actual population at 14,931-14,981. After this is achieved, set food to positive and allow it to grow back (hopefully, one turn) and ship the newly assimilated citizen to the low-population world! You can do this several times in a row resulting in faster population growth after the population has been reduced by 3-4 and you don't have to wait on slow traditional assimilation to get extra people where they're needed.
Another benefit is a lower population isn't as likely to overthrow your garrison. Try to make marine count meet or exceed half the total population of the planet. That is, have five or more for a population of 10, or six or more for 12, etc. Sometimes the game won't let you do this by unloading as many as desired, so starvation is a way to accomplish this.
However, be careful. The game usually starves the last conquest, but not always!
This tactic will allow expedited starbases after factories and pollution controls (if needed) are built in newly conquered worlds. Voila! CP problem solved, or at least greatly diminished which can help to prepare phase two - capturing the rest of their colonies.
Choose the victim if possible. Tolerant citizens are the best to go for — they build the quickest because they don't make pollution. No need for Atmosphere Renewer.