My first runthrough is a warm-up on the lowest difficulty and it's going moderately well. There is a lot of complexity to this game and it's taking me awhile to remember the nuances of how to play. Basically, you have to build up your reputation with the various alien species, but since they start out as rivals, actions that benefit one can aggrieve the others.
Also, while you're focused on your own goals, all of the alien factions are focused on theirs, so there are difficult time-management decisions to be made, in addition to the complex political wrangling. Spending 10 months building a special diplomacy-enhancing building on an alien world might help you out in the long run, but while you are doing that, you are out of commission and thus unable to respond to a war or an outbreak of disease.
I've already got my basic Federation set up, thanks to the fact that three of the species are naturally peace-inclined and thus easy to ally with each other, but once that early hump is cleared, it becomes a much steeper climb to the top. The various species bring their rivalries and prejudices into the Federation and the enemy of one becomes the enemy of all.
I think I've got it basically figured out. Focus on one group at a time. Intervene in wars to prevent the non-Federation species from gaining too big a power base, and buff up my Federation planets so they don't have to worry about invasion. I'm pretty sure that at least one of the species is going to go extinct before all is said and done, but then again, this is easy mode, so maybe I'll manage to pull it off.
I'm looking forward to giving it a shot.