I've been using what I think are pretty balanced forces in my Chosokabe campaign. Having conquered the southern Islands and now Honshu just beyond Kyoto, I have 4 and soon 5 full stacks. I'm just now building "consolidated land" farms and am running a 4.5-5k profit each year.

Starting with predominantly Ashigaru forces, I now use a roughly even split. The Ashigare are what I put in harms way most, Samurai are only commited at critical points of the battle. As Chosokabe I evidently have a lot of archers so my usual army is something like:

General
4 Samurai archers (deployed behind spear wall)
4 Ashigaru archers (deployed as skirmishers, falling behind the spear wall as combat is joined)
4 Yari Ashigaru (my "fixing" force)
2 Yari Samurai (deployed behind and to the side of the Yari Ashigaru line)
2 Naginata Samurai/Monks -or (rarely) Katana Samurai
2 Yari Cavalry
1 (either) mangonel or bomb throwers

Usually, the AI will charge me. I ensure this by either campaign map positioning of my force to fight a defensive battle, or prompting the enemy to advance by engaging and withdrawing my skirmishers. A mangonel can help prompt the enemy to attack from further away.

The objective of my tactics is to draw the enemy into range of my samurai archers and onto the spears of my Yari ashigaru wall. Usually the mellee doesn't go so well for the charging units which leaves my Yari Ashigaru to charge the enemy archers (who normaly stay behind their melee infantry as it charges) while my Cavalry and Yari Samurai flank and engage them before running the routers down (which my general also gets involved in).

The most challenging battles i've had are when the enemy force consists of more archers than mine and/or the enemy army refuses to engage. In these cases, I try to focus my fire on a portion of the enemy line and roll the line up with my Naginata, Yari Samurai and cavalry.

I've yet to even recruit matchlock troops but I can't imagine having more than 2 in the above line up. I'd probably swap them for some Yumi Ashigaru.

Bomb throwers are fun, if often dangerous to my own troops. I've mostly had them standing right behind my Yari Ashigaru line so that their minimum range is beyond the front of my line. I'm not sure how much this improves the safety of my troops in practice...

I also try to swap out mangonels and bomb throwers in my armies that attack and defend castles. Mangonels are great in siege attack, bomb throwers excellent in siege defence. Mangonels slow my armies down but are more useful to my open battlefield tactics.