S3TC question

Hi guys!

I wanna ask something.I have a data in which all bytes shoud represent a green index on RGB data.I use GL_GREEN for format.But the question is.I wanna compress that data with S3TC.Is it possible and safe to use GL_COMPRESSED_RGB_S3TC_DXT1_EXT and it’s RGBA eqvivalent as internal format while the format of the source data passed to OGL is GL_LUMINANCE/ALPHA/LUMINANCE_ALPHA/RED/GREEN/BLUE or shoud i convert it to RGB/RGBA myself?

According to the spec, the driver should be able to convert any supported input format to the internal representation, assuming certain criteria are met (that are documented).