The most famous and definitive strategy guide for SMACX is Vel's SMAX guide, by Velocyrix, aka Chris Hartpence. It's over 200 pages long and covers pretty much every concept in the game.
In addition to Vel's guide, the forum collects some smaller articles on strategy, but some of the early texts are missing. Perhaps lost in the apolyton shutdown? It is the official opinion of the technical administrator that humanity will never quite recover from this loss.
The ever-useful The SMAC Academy also hosts some words of wisdom.
- Don't forget the ship's intrinsic ability to use Long Range Fire! In games where defense techs outpace weapon techs, Heavily armored transports may sometimes be all but impossible to stop with the ATK your current tech affords. Since artillery duels don't consider DEF you may bypass it altogether. It is also helpful in the opposite circumstance: You've spotted an enemy from afar, your hull is paper thin and you do not have the move points to make a get-away or close in; using LRF will at least give you a chance.
- Where ATK strengths are disproportionate, never use your last move point in dangerous waters. You may wish to advance at a fraction of your normal speed, peeking forward into the fog of war and retreating if there are no targets. For example, using a cruiser with six movement, you might go forward four times, then back twice. If they are within your move range, you may attack first. Otherwise, you have given yourself a safe buffer to check next turn.
- A fully armed and armored cruiser may be expensive due to the way unit cost is calculated. You may opt to use foils for your fleet instead, using only one or two cruisers to scout ahead.
- It may be worthwhile to use cheap, skeletal hulls as scouts. Use Patrol to increase your vision coverage each turn and catch enemy invasion fleets before they're bumping up against your shores, then gather your fleet for an ambush.
- Planetpearls can be a significant source of income. If you've caught any mind worms, you may take advantage of their high movespeed and lack of maintenance in fungus to roam about looking for weaker boils. This is especially effective during Perihelion.
Terraforming / Resource Management
- Supply crawlers, despite requiring no maintenance, are limited in that they can only harvest one of the three resources from a tile at once. Rocky squares, which are mineral rich but may not yield any nutrients or, energy beyond what rivers provide, are excellent candidates for supply crawler sites, and generate less Eco-Damage to boot. Indeed, there is little reason to have the square worked once Industrial Automation has been researched.
- Ground transports are, at first glance, a pointless use of the transport module. However, they have niche uses in which they provide effective support. A speeder chassis may only travel three squares along a road before it suffers hasty penalties, but if it is transported, its effective coverage doubles to six squares. This is a four-fold increase (9->36) in area for double the maintenance cost, or none at all with clean reactors. The increased range guarantees the first strike when attacking an enemy city.
- Similarly, infantry "legions" with full armor and weaponry may enjoy the movement benefits of speeders without being prohibitively expensive to construct. See: Unit Cost (Advanced)
- Your valuable, wounded Elite units may be whisked away to a friendly city where they may quickly repair to full hp, without being hampered by the movement penalty for injury.
- Air transports are the safest way to move units through hostile waters, requiring not even an escort, and freeing your military to pursue other tasks. Assuming, of course, that one doesn't linger within range of interceptors.
- Ground units may disembark from an air transport in mid-flight (i.e., outside friendly bases) by using the Unload command. Simply clicking the (L)oaded unit does not work. Unlike air-dropped units, they suffer no damage when deployed this way, allowing one to deliver units right to the enemy's doorstep, fully prepared for combat.