Source for Chinese sword information and typology

What you ask for isn't readily available, because you're asking "why". Actually, it depends on which "why" you are asking. There is a purely physically functional "why":
1. Longer swords give more reach
2. Broader thinner blades are better for cutting soft targets
3. Thicker stiffer straighter blades are better for thrusting
4. One-handed blades can be used with shields
5. Shorter blades are easier to carry as secondary weapons
but this is pretty universal, and the same answers apply for swords from other places.

It doesn't tell you why functionally similar swords like yanmaodao and liuyedao changed in popularity from one to the other, why officers and militia might carry jian ("jian = high class" fails for militia jian). Pre-Song (maybe pre-late Ming) we have very little useful in written sources, especially in sources that have been translated. There are military manuals with catalogs of weapon types, but these often just briefly describe the weapon, and perhaps its virtues.

There are similar problems for Europe, and other places. Late European military swords are reasonably well-known, and the reasons for adoption of new swords are sometimes known. But older stuff is less well documented. Before the British adopted standard patterns, British/English forces would be equipped according to individual whim (either of the commander or soldier), with no reasons for their particular choice known today. We can guess, from functional considerations, what the swords might have been good for, but that doesn't always answer the why.