In most my masochistic games I've seen Drengin (and now also Korath) playing by that rule, and if they're not hampered by a bad start, or a close neighborhood of both Altarians and Torians, they just roll over their neighbors, one at a time. And while they're taking planets, they also receive techs from their victims. So at the end they really get everything. When they get their momentum (planets of one or two races), seems that I am in most cases the only one that can stop them. Also in most cases not by destroying their huge-attack fleets, but by attacking their economy - planets and galactic resources.
IMO having good weapons (and all else below average) shouldn't give such an advantage. Also neighbours of Drengin should react to their presence by researching better weapons and proper defenses, instead building infrastructure Drengin will use. Let Drengin remain an aggressor, but please change the game in such a way they'll have more problems with their approach. Like:
- made maintenance costs of ships higher,
- cripple somewhat drengin's research capability,
- decrease morale bonus Drengin get by default,
- made close races react better to Drengin as neighbour (war is imminent),
- made all races react better to anyone building big military. It's silly that they like them more instead fearing them more.
You might dislike my proposals, but I've seen that happen in way too many games. I'd just like to se more variety.