[Hunpecked]

Bear in mind I'm still fairly new to the game and have only played against AIs. My advice assumes a simultaneous 2000 point game, large quadrant, normal tech start.

To counter the AI Tessellate without taking Religious or Crystallurgy, I'd suggest taking Hardy Industrialists, increasing Construction, Mineral, and Intelligence (research) traits, and reducing Maintenance to the bare minimum (5% per turn). This should allow the human to take the lead in weapons research, build and maintain a fleet at least twice the size of the AI's, and hopefully cripple the Tessellate before they get the Talisman.

I'm skeptical about countering the Tessellate with seekers/fighters. In theory they can overwhelm an enemy, but when the AI runs the battles they tend to come in waves rather than all at once. In a recent battle my 1 heavy and 2 light cruisers (8 PPB and 8 PDC total) defeated 3 Aquilaeian light carriers with 60+ fighters; no more than a dozen or so fighters came in at any one time. The Tessellate ships I've encountered so far seem adequately armed with PDCs.

I'm currently playing (slowly) a simultaneous 2000 point game against 5 of the top 6 Deathmatch III AIs (TDM Modpack 3.40). I play the Terran Confederation with modifications, e.g. Strength 45, Research 120, Minerals 120, Construction 120, Maintenance 119 (i.e. 6% per turn). I took Advanced Storage, Hardy Industrialists, and Propulsion Experts; culture is Merchants.

Tactically I've had good results against the AIs starting with seekers, then moving to PPBs and now to APBs. Of course, most of my tactical success (even against Berserker races) is due to a substantial lead in Combat Sensors and ECM, plus fleet training (and to a lesser extent ship training). However, my most devastating weapon by far has been the mine; in SEIV apparently one can't program a good anti-mine strategy for the AIs. At one point over half my frontline systems were held by minelayers alone while my fleets attacked elsewhere.