Can a chainmail speed penalty be more than -1? If not, isn't Chainmail Agility inferior to the Fleet-Footed feat (+1 feat bonus to your speed)? Do these feats stack?
If you're using chainmail, Chainmail Agility & Fleet-Footed essentially do the same thing. Fleet-Footed is slightly better in that you still get its benefit if you for some reason take off your armor, and unlike Chainmail Agility it helps dwarves (who always ignore armor penalties to speed).
Their effects do stack; Fleet-Footed gives a feat bonus to speed (and therefore does not stack with anything else that gives a feat bonus to speed), while Chainmail Agility reduces a penalty rather than giving a bonus.