AFAIK only the vikhr are beamriders, while the S-25L and Kh-25ML are "conventional" home on laser spot missiles. But as stated above the laser system are unlikely to be compatible.
As for Ka-50 lasing for Su-25, the Ka-50 has a dedicated laser spot illumination mode and IIRC you can buddy lase for another Ka-50 firing Kh-25ML missiles (although I dont think these can be employed from a Ka-50 in real life).Therefore, it should be also possible, although not necessery implimented in DCS, to lase targets for a Su-25 fired Kh-25ML from a Ka-50