Images rendered from 3D scenes created in an application such as Maya are typically scene-referred linear RGBA images in a floating-point format such as OpenEXR.
To composite these images in a scene-linear working space such as ACES, you typically need to use one of the transforms in the primaries/ directory to convert from the native primaries, for example, from the Rec. 709 primaries.
If the linear CG images have incorrectly had a gamma applied but have not actually been tone-mapped, you can use one of the transforms in the gamma/ directory to remove it first.
The alpha channel represents the pixel coverage and should remain unmodified by any transform.