It depends what yur armor units are good against. If you build a squad of anti tank armor units, a squad of anti armor infantry can wipe yur squad out.

best defense is not the shield

it's activity

I personally usually keep 2 squads inside my colony, or close to it. And do usual hive cleaning - killing every single units from neutral or used-to-be-hostile colonies in hive.

You're missing the point, armor units are easily killed and inflict hardly any damage. For keeping your colony or anything you own safe armor is the worst option.

Infantry is easy to manage but is not the best for defence...it depends on the person..so if u r worried abt cost then infantry is best....
On the other hand if you see heavy units then they r pretty good for defence,2nd is cavalry and 3rd is infantry..
infantry die fast and kinda like absorb the impact..
cavalry have a bonus on heavy units but ranged heavy units wont let any thing come close to it if you have some other troops..
the best for defence is all of them combined!!

this is a bad defense for the same reason as it would be a bad offensive force

I suspect that the reason a lot of players have lots of infantry in their hive, regardless of what chassis they are mainly using, is to save up oil.

you can buy 3 infantry armor (the cheapest forces to buy) for the price of 1 tank armor, but if you put those 3 infantry in separate squads on maximum delayed launch you can save up 3 times the amount of oil as you would with 1 tank.

hope this makes sense

just my guess anyway

Multiple things of this post are untrue. Infantry could actually arguably be the best defense as they have no weaknesses to other chassis. Also multiple chassis is always a bad idea.