You cannot sit on 4-6 mexes. Should you do so, you will not live very long and you will
stall mass hard. The key to playing and winning both ladder and team games is to expand, secure expansions, build more units given resources coming from extra mass extractors (later as "mexes"). If you have mass and energy (mass from mexes in your main base and on your expansions, energy from hydrocarbon power plant (if you have a spot, later as "hydro") and power generators (later as "pgens")), you can build tanks off your factories. Those can be used in combination with radar to detect incoming raids that target your expansions and then you can divert your units to beat the raid and keep the expansion.
Should you desire to not worry about your expansion, just put a small garison and a T1 PD there, as well as radar and check up on it from time to time.
If you ask "But where do I get mass early on when I have no expansions?", I answer:
Send your first engy to build hydro if there is one as you will need power, send next 1-2 to the closest mex clusters to build them. Then send 1-4 engies to reclaim (Select factory and alt-right click on trees/stones/wrecks) so you have some mass income besides your starting mexes.
On 5x5 maps, do not do air first or second. It is safer to do air third, fourth or even fifth. On 10x10 you can do 2nd air if you have hydro. On 20x20 you definitely need 2nd air, whether or not you have hydro (just do enough pgens before building air factory)
If you wonder how to set up land spam, figure out what your army needs depending on your faction and what your reconnaissance tells your about your enemy. If he is making PD - alt-right click out of a factory and loop mobile artillery/mobile missile launcher (if you are at T2 (later as "arty" and "mml", respectively)). If he is upgrading to a higher tech, either storm him fast while he has fewer units or get the same tech to counter your foe. If he is making a lot of units of the same tech as you are, you can make same or more to win OR transition to a higher tech. Should you choose higher tech, ensure that you have enough units to hold off your foe.
That is when PD comes into play. If you want to rush a tech, ensure that you have enough lower tech units to hold off your enemy. If you are not confident of your capabilities, you may elect to build static defenses to slow down your enemy and buy you time to rush tech. Usually players tend to not build arty unless forced to do so by enemy PD. So until it comes and kills all your PD, you can get several minutes to get the stronger tech and build some of those shiny, new and powerful units.
That is my advice. Suggested spam build orders are in the spoiler.
T1 land
UEF - 5 tanks + scout
Seraphim - 5 tanks + scout
Cybran - 5 mantis + scout
Aeon - 3 tanks + scout (because range and auroras need radar
T1 air (all the same)
5 interceptors (AKA "inties") + scout + bomber
T1 navy
3 subs 1 frigate or 3 frigates 1 sub, depending on enemy naval composition.
T2 land
Just take a tank/assault bot (seraphim), sequence it up and done. Do so on many factories and do not forget about flak, reserving at least 1 factory to building it.
Do not know the rest because my ladder games rarely last into late T2 and T3 stage
