Let me first note that lowering agility with a constant value can lead to some rather unintuitive results. Let's say we have a light and swift warrior with 100 agility and a strong, but slow, warrior with 60 agility. Both of them equip an armor that lowers agility with 50. The light warrior is reduced to half agility, but the strong warrior is reduced to a mere 1/6 of his agility. If anything, you expect a heavy armor to affect the swift more severely than those who already are slow. Consider requesting a script that changes the armor modifiers to a percentile system i.e -30 agility on the armor means -30% agility.
Anyway, let's start with light armors. There's no real point in giving them a bonus to agility since the defensive value they offer is enough to make them a superior choice than not wearing an armor at all. As for giving them a penalty, it depends on if you want the player to sometimes choose not to wear them at all. If yes, then you should give light armors some penalty and if no, then you should not.
Now for heavy armors. What agility affects is turn order, chance to escape, chance to evade attacks that allows evasion and chance of receiving a critical hit (the more agility you have, the less critical hits you eat.) I don't think you really want to fiddle to much with chance of escaping. Questions such as "what is a reasonable chance of escaping from a battle if the player gives three out of four characters a heavy armor?" doesn't seem like something you want to spend a lot of time considering. The chance of evading due to your agility (as opposed to due to your actual evasion stat) and the chance of eating a critical hit is rather small unless you do something to extreme. As long as armors don't slow characters down to less than 50% original value, maybe 40% for really heavy armors, you're pretty safe from disruptive results.
Turn order probably is the main issue here. It's fairly simple in theory, just think of how severely you want heavy armor to affect when a character wants to move. Let's say you give the fastest character a heavy armor. How many of the enemies do you now want to be faster than him/her? If you for example want the otherwise fastest character to be slowed down so that half the enemies now are faster, then the heavy armor simple has to alter agility so that it's lower than half of the enemies in the current dungeon.
One other thing though, do not use low stats for this. An agility penalty becomes less noticeable the less agility the character has to begin with. A character with 30 agility will never act before a character with 60 agility, but a character with 3 agility will very often act before a character with 6 agility. Even at just 1 agility, you still have a chance of acting before someone with 6 agility. For safety's sake, the slowest character should have at least an agility score of 30 at level 1.
