Ivanova was outnumbered in her battle with the Shadomega. She said it and the fact that the only way slower ships can attempt to sorround faster ships is when there is more of them. Plus it says it on the DVD synopsis.
In the B5 universe there seems to be 2 types of space strategies. One is using huge destroyers with supporting fighters (EA, Narn, Minbari). The other is using medium class warships (Vorchan, Sunhawk, Whitestar, Drakkh "fighter"). My argument is of the inherent superiority of the latter. They can be built in greater numbers, pack quite a punch, relative to their size, and are more manueverable.
I beleive that fighters are mostly used againsts other fighters. There are a few instances that fighters take on and destroy huge warships, but they are still relegated into a supporting role. (they are used for attacks if there are no other options available). In the show we have seen Vorchans, Sunhawks, and Whitestars as the main element of an attack fleet.