Yes, it is true that the SIG 226 Mk25 and Combat models both share the M1913 rail (or at least very close) which is flat bottomed and a bit wider than the accessory rail on most of their other 226R models.
I doubt that there are any holster makers currently offering a specific holster for the Mk25...it is too new...unless you go with a custom holster. I'm not a personal fan of accessory rails, so I don't have a lot of hands on knowledge. However, I will mention that certain plastic holsters have some extra room in them, under the dust cover, that may work for you.
My first thought would be to check out the Blackhawk CQC Serpa. The one I've used a lot, with a 226ST and 220ST seems to have enough room to accommodate a wider dustcover. You might also check out the Safariland offerings, both the SLS (rotating hood) and the ALS (ejection port block) retention holsters might work. In either case, a call to CS might help...I don't think they'll really be sure either (it is a pretty subtle difference)...but, a better idea would be to go to a store and try them out.
As a fallback plan, you could look at holsters designed to carry the gun with a weapon light attached...they'd have more room to accommodate the light attachment.
I really like Tom and Michelle at Top Gun Supply and have only had positive experiences doing business with them...they are my default recommendation for SIG parts...but I think there are better choices for holsters than the iTac line