Hi!
To be honest I selected rare for everything.
How many planets have other players? If most of them are restricted to their home systems and maybe 1 or two more planets, you could still have an intresting game, esp. if you're playing below masochistic. With Humans a viable path would be an alliance victory.
I am thinking of concentrating on trade, diplomacy and building up my military
Excellent approach. With small number of planets trade can really make a difference in income. Just make sure you "hit" the farthest planets possible.
When Earth is maxxed (everythign built), switch to research and build military ONLY if others with big military start seriously threatening you. A few fake defenders can help here.
Research path: level 2 buildings, 1st trade (immediately build traders), some military tech to be able to build fake defenders, or 2nd trade (depends on the situation). When trade starts bringing money go for 3rd level buildings and last trade. If there are big extreme planets available, consider researching the tech that will give you the most tiles. Otherwise go for terraforming, or (if wars already started) for planetary invasion (see below). If you're in danger go for medium hull, missiles and defenses for weapon your main adversary is using (level that gives 2 defenses/item).
Watch actions in galaxy constantly. Drengin and Korath ares especially dangerous to grow big. If they attack someone and he's losing, consider invading his planets before Drengin does.
Playing Humans should help you quite a lot. Try to befriend with everyone (trade routes will help), have at leat moderate score in military and soon you'll find some races as "close". Trade away Alliances and ally with them and ride that path to the alliance victory.
BR, Iztok