Ooops. I am not flowing with the river here, as I play without adding more leaders, just the starting one and any coming my way during a game. Leaders, with celebrities (I play synthetic civ, zero need for morale/food), are the 2 types of citizen I never ever train !
Leaders are useful only on long games, many planets (+10?), you don't specialize your planets, and if you have the combination [many planets] + [long long game expected]. But as Commonwealth is too useful when you have many planets, and leaders are not productive/efficient enough if you ramp up the difficulty: You need to specialize more. Besides, they won't progress.
Choosing your citizen is not a 1 rule. It depends on your own style; the type of victory you are looking for (or type of punishment...); the layout and opportunities of a map, the difficulty; and the civs you have to compete with.
It depends also a lot on your luck ! Say, you just generate a map that gives you a Precursor World with bonus to production, ship building, research, or wealth: you really want to maximise the benefits with a citizen able to boost this world !
The last 600 or 700 hours, I only play godlike, with my customized civs, and it can go up to 82 civs at once. Or with less civs, but opposite ideologies (ex, I play Benevolent alone, versus only Malevolent civs). Large to immense maps (but my PC is too weak now for having a smooth game).
The settings of your map will also guide you for the citizens. I play without brokering researches (always disable it on high difficulties!), slow pace, slow research, rare resources, synthetic race. Research will be a priority, as others will have 200% more than you, get random techs, and reach Ascension time with harpoons when you hit turn 50 or 80 ! ...
In this case, it is a game of patience, as the AI is a bit one-minded at godlike, and around the 100th turn, you will have half the research they don't have, and trade trade trade... for buying mercenaries that will be your only 1st defence. No good ship without a citizen-commander !
It is a special gameplay as you do NOT want to expand your civilization with numerous planets, at least until you are sure you can survive a very strong combined attack... You want to stay under the radar, develop a LOT your diplomacy (researches, ideology, and especially trades/government), avoid any unnecessary conflicts until you are able to produce a just half-decent fleet (always with customized ships).
I try to have 3 or better 4 planets, stick with them, specialize 1 in manuf, 1 in research, and the 2 others (often small), as supports for production/wealth/diplomacy-influence. No expansion as it would mean more border conflicts (and -1 or -2 anger), and impossibility to defend your worlds.
It is then an equal share among Scientists, Engineers, for the first 4 or 6 citizens, then the 5th or 7th is a Commander, and doing the researches for being able to do it also in this order. Then, step by step, it becomes 2 commanders for each each scientist and engineer.
I can also take, around the 80th turn, a diplomat and a spy (for the spy, I prefer to research it and gain 1 : it opens the useful espionage screen). The diplomat goes either to a planet with a lot of influence or designed for it, or with the most powerful civ, with the worst <<< "bad temper" arrows...
Generals? Only if given by the game (or by a another civ when having their planet or giving it to a Commonwealth).