Naa bro, the zodiac signs only effect how you will react with other characters. Like if you have Best compat you will do more damage to each other, items like phenoix downs will revive with more hp, and ect.

As for the Sand Rats Celler fight, I'd recomend having Algus and Delita both as knights, and giving them the best armor and shields you can find. Everyone else should try to support them. Algus will take one side and delita the other.

In my experiance, delita will get bombarded alot less so it's usually Algus who will need more help. Still, having weapon guard and some shields will really make a differance and you might be able to get some lucky defences and make them a bit more tanky.

Support units that would be nice would be:
Archer: Only if you have the long bow.
Black Mage: Provided s/he is defended from arrows.
Chemist: Duh.

Also, remember, just cause the computer ai for delita and algus does stupid stuff, doesn't mean you have to aswell to go and save them.

Hope it helps, good luck and Godspeed.