Is my Animated Knife (Construct, Diminutive) too strong?

[Zombimode]

I need to give it 2 feats and it has 14 skill points to spend.

No, as a creature with no Int score it does not gain any Feats or Skillpoints through hit dice.

I also would like to require a reflex check to grapple the knives and cleavers, of DC 15, since they are so small and quick. Is there a way to that within the established rules? Is this fair to my PCs

Grapple attempt already require a touch attack to get a hold for which this creature has an AC of 18 (because it is so small and quick).

I'm aiming to pit 5-8 of these against a group of 5 level 4 PCs with the hopes that they will lock them in cabinets or drawers as an alternative to a strait fight.

Depends on the group. One point to consider is that the party probably can't outrun the animated objects with their 40 ft. fly speed.

Also if I give it a Str of 1 and Weapon Finesse as a feat, it will retain the +7 AB but lose out on a lot of damage...

For the record, the Strength score of a diminutive animated object can be extrapolated to 6. But since your knifes have 4HD, Str 18 seems not out of the line to me.