Well, if you are going to use the Sagittarius, I would suggest throwing in the Oracle as well for the scout redirects (plus an extra missile).
It depends. One thing I see quite often is people buying scouts in fairly small games, and they never really pull their weight.
Remember that a scout gives you a 50/50 chance of getting a redirect - that means it's effective 'firepower' - that is the number of extra rolls to hit you'll get - is 50% of 50% of the number of missiles it's trying to redirect.
So if you've got a just a saggitarius and the oracle's own fairly puny rack, it's probably not worth it - you might well be better off with a second saggitarius (especially since ships can use
Concentrate All Firepower! and get the same effect). If you've got a small squadron of
Tethys then it gets more worthwhile.
The best reason to get a scout is probably taking lots of assault fighters. If you have a big pack of
Tiger-class Starfuries, then you can get quite a few rerolls with their smaller missiles - although when I say 'a big pack', I'm meaning multiple
Avenger heavy carriers.
If you put them in a line it is easy for your opponent to just go along the line blowing them up one at a time, and not many of your own ships will be able to return fire. Just put your ships in a nice formation, I almost always put them very close right at the start even if I lose initiative.
Good advice. A nice, solid 'phalanx' of ships is a daunting prospect, especially with the 'secondary guns pointing every bleeding which way' designs of the EA. Keep out of explosion distance of one another, obviously, but if ships are in mutual plasma cannon range of one another:
(a) you can use otherwise idle secondary guns to shoot at fighter flights attacking friendly ships.
(b) it becomes a lot harder for races with longer range secondary weapons (which is just about everyone if you're EA early years) to attack you but stay out of your own range, because overflying one ship means running smack into the guns of another. Olympus, with their railgun and missile turrets, are extremely good at this and make for great flank protectors.