Small, medium, large sounds good to me.

YES!  I have thought the exact same thing!

If it is one ship, it should say something like "Hostile scout incoming"

Several ships it should say, "Hostile ships incoming"

Tons of Ships, it should say,  "Hostile Fleet Incoming"  (with a little urgency in the inflection)

In defense of the developers, I think this is a little tricky to implement, because ships don't all warp at once, and the way the logic probably works is to announce on the first ship and not re-alert unless the phase lane clears.  It isn't impossible though, and would certainly be a nice touch.

This is a great idea - I too am having a war on 3 fronts between 3 different factions out of the other 9 and 1 blasted scout is what the planetary defenses are for - I keep having to pop up from micromanaging the fleet to take a look and then pop back. It's only a few seconds, but whe you're trying to tell your bombers to go around the flak to take out the command cruisers and the LRMs to take out th erepair ships and have the capital ships keep the cruisers busy even a few seconds matter in a fight that you're outnumbered in where every ship counts.