Often the main series games gives wild pokemone or pokemon that own that will have some advantage over the first gym leader (unlike gen 1 and FL LG where if you pick charmander you're screwed unless you find a mankey). Gen 3 mitigated this by giving the fighting type to combusken, allowing you to use basic fighting moves against Roxanne (or using the lotad/seedot) and gave you ralts, zubat, tailow, wingull, sableye, and shroomish (with effect spore) before the second gym, etc.
For instance I'm doing a washington state pokemon game and the first leader is a dragon type. I'm sneaking in a very rare Goomy and cubchoo in the temperate rainforest you have to navigate. (Cubchoo has an environmental reason; polar bears are fishing in the rivers of northern Quebec now due to the receding ice.) They're both fairly weak and evolve until late in the 30's or 40's, making them a perfect early game treat (and the other pokemon in the game will quickly outclass them...unless you are patient). For the second gym (a fairy gym) I'm also making a normal/steel prairie dog and sneaking roselia and venipede before the second gym across routes 2-4 to give smart players an advantage. However, the third gym leader would be much more difficult, etc.
It is a matter of placing the good and useful pokes carefully. If you place too many useful pokes too early on, then you've made it too easy and the opposite is true: if you don't place enough helpful mon's then it would be hard only because of bad design.
You also want to allow the player to obtain some basic pokemon to begin formulating his or her team. XY gave the widest variety at the start, I think, which allowed for pretty unique teams early on. Since normal isn't super-effective against anything it seems to be a great introduction type with more complex types later to be introduced. Unleashing the full 18-type chart at once would be ridiculous and overwhelming to the new player. However, by the fourth gym usually a full variety of pokemon have been seen (minus a few types, usually ghost, ice, and dragon). By then the player would be fully acquainted with a large variety of pokes as well enough to make a stable team and the normal types you would have collected at the beginning would start to be outclassed by the others you encounter, anyway.