In multiplayer, commanders and experience mean
almost nothing. These do have a meaning in skirmish though, but still relatively little compared to the Campaign (which is off topic).
But I'm trying to figure out a general guideline, because it is also written in the guide that there is no need to learn the entire research tree by heart... In any case in the game I keep forgetting all the pre-requisites of all the weapons I like. This is why I asked the elements question.
Machineguns require mg damage upgrades, assault guns require three rof upgrades, TAG requires neural synapse as well, hurricane requires tmg, whirlwind requires AG, some of AA gun upgrades require a lot of cannon or rocket research (!!). Nice cannons require damage upgrades, HPV requires 1st cannon accuracy, AC requires 1st cannon rof, railgun requires hpv, neural synapse, and 7 cannon damage upgrades (out of 9), while further cannon ROF or accuracy upgrades don't lead anywhere. Flamers require flamer damage upgrades, plasmite requires research upgrades as well. Lancers and tank killers require rocket damage and accuracy upgrades, scourges require tank killers and neural synapse; ripples, apart from MRA, require CB tower, which relies on first mortar accuracy upgrade and first sensor upgrade. Lasers require neural synapse mk2 and sensor upgrades only. Mortars require one cannon damage upgrade, bombards require one alloy upgrade as well (!!), howitzers require mortar damage upgrades, some howitzer upgrades require a lot of cannon damage research (!!), bomb upgrades and heap bombs require second mortar damage. Thermite bombs require inferno damage mk2 (while mk1 is enough for inferno itself, and inferno itself isn't required for thermite bombs). Cannon fort requires a lot of cannon damage and wall upgrades, you'd have to research second engineering upgrade at some point; rocket forts require rocket upgrades (a bit more than necessary for the TK) instead of cannon upgrades. All ROF upgrades rely on first production upgrade, all accuracy upgrades require synaptic link. All alloy upgrades require research module. Better bodies are unlocked by researching alloys, heat armor and engines. Tiger requires better alloys, while Mantis requires better engines, hovers and vtols require 2nd and 3rd upgrade respectively. Satellite stuff requires all possible sensor towers and sensor upgrades and full research upgrades, auto-repair requires neural synapse mk2. Borg alloys and heat armor is separate from tank alloys and heat armor. Superborgs require a lot of borg alloys.
I think that's close to as much as i remember during a multiplayer game (written it without consulting the guide right now, and i'm pretty sure about everything i wrote, feel free to correct me). Things marked with (!!) are seriously weird and unexpected, and i think it's a complete list of them; apart from that, there are no such heavy cross-line interactions, every weapon line lives on its own, so it's easy to remember most of the stuff.
The early-game tech tree is pretty much to be memorized (until the three modules are available, for example, and how to get the basic early weapons).
Try learning these one by one. For instance, try to practice producing MRA cobra hovers in under 12 minutes (from T1 no bases with 2 trucks at the start); it's one of the common multiplayer strategies, specially in FFA where you can bet on having some time to tech up.
That's it for the research tree, i'll cover what upgrades affect what some other time (: