First, to directly answer your question regarding the specific encounter you quoted...disciples use 4 fleet supply while cobalts use 5, so in theory 5 disciples should be equivalent to 4 cobalts (in this case the theory is pretty close to the reality)
As for other things, here are my thoughts...
In pure LF fights, Cobalts and Disciples are close enough where other factors are going to be far more important...IMO disciples are better because they have more DPS per fleet supply and thus are better than cobalts at focusing on SBs or caps...as far as corvettes are concerned, TEC is the clear winner...this is why a cobalt+corvette fleet is stronger than a disciple+corvette fleet...the LFs are basically a wash, but the stronger TEC corvettes gives TEC the best raw fleet power in the early game...
Just to reiterate a point made earlier, LFs generally beat corvettes, so you (as TEC or Advent) really only should bring out the corvettes if you anticipate LRFs being brought against you...that said, if your opponent goes cap heavy then corvettes are better than LFs for that...
In small numbers, illuminators are still countered by corvettes...its the medium or large amounts of illuminators that are really troublesome and difficult to counter, and malice really nullifies any corvettes trying to counter the illuminators...Vasari corvettes drop like flies to illuminators and advent corvettes don't fair too well either...TEC corvettes aren't awful against illuminators but you would still probably be better going with other fleet compositions...
For TEC and Advent, you are almost always better trying to spam LFs before investing in other unit types....if you see your opponent bring out kanraks or LRMs, corvettes are the obvious ship to incorporate next....if you see your opponent start to bring out illuminators, TEC and possibly Advent corvettes will still work well for you....if your opponent has already amassed illuminators, then you are better building LRMs or your own illuminators...
If the Advent player has any combination of disciples, illuminators, and defense vessels (LF, LRF, and flak), then pure LRM or pure illuminators will work pretty well...your main advantage will be in your ability to kill SBs or caps...if however your opponent is going more an illuminator+corvette route, you will probably have to incorporate flak ships especially if you are TEC...
If you are Vasari, kanraks are probably your best bet against illuminators (assuming PM upgrades) but honestly Vasari frigates are pretty bad all around so hope you have some other advantage (economic, orkies, etc.)...
It's not uncommon for the deciding factor to be whoever micros their ships best...illuminators and flak require good micro to make sure all weapon banks are firing and that micro is often more relevant than the actual stats of the ships...
A general rule of thumb is that on a per cost basis, LF+corvette beats LRF+flak while on a per fleet supply basis it's the other way around....so basically, once fleet maintenance starts to nullify the per cost advantage of the LF+corvette fleet, it's probably time to change your fleet composition...
One detail that often gets overlooked...on a per fleet supply basis, illuminators and kanraks build a lot faster than LRMs...this means that if a player is getting fed or has a decent income rate, they are going to be able to replace their illuminators a lot faster than you can replace your LRMs...this is an important consideration that in some instances can be more relevant than the actual combat performances of the ships....