Reduce person abuse

[Gruftzwerg]

As alternative idea:

How about giving them the Slight Build trait from the Kobold Web enhancement?

The physical stature of kobolds lets them function in many ways as if they were one size category smaller. Whenever a kobold is subject to a size modifier or special size modifier for an opposed check (such as Hide), the kobold is treated as one size smaller if doing so is advantageous to the character. A kobold is also considered to be one size smaller when "squeezing" through a restrictive space. A kobold can use weapons designed for a creature one size smaller without penalty. However, the space and reach of a kobold remain those of a creature of their actual size. The benefits of this racial trait stack with the effects of powers, abilities, and spells that change the subject's size category.

While this is not the same as the ability you mentioned, it is a "officially balanced" racial trait. It even effects combat effectiveness compared to your ability.

Imho when I compare your ability to the Kobolds ability, i don't see any crucial abuse potential.

The slight arguable edge your ability would have is:
- being tiny lets you move through occupied squares by enemies (still provoking AoO)
And it is limited by combat start as you said.
Not such a biggy imho, since most of the time getting access to some kind of "reduce person" (permanent or magic item) is not that hard to get.

Imho go with the Slight Build or your own ability, as you see it fit.