"Clan Mechs require specialized gantry systems in order to conduct the limited customizations available to Omni mechs and therefor cost more as they come with all the equipment needed!" You heard it here first folks!
I don't know how they will do it, but they would definitely allow owning both Clan Mech and IS mech. If not they would run into trouble for people that are buying the package , at least when it comes to the already owned Hero mechs. I have a feeling that the Clan Mech is expensive when it comes to they pricing though. The Clan Mech package currently re reidculously expensive, expecially when you compare them to the Phoenix Project. If that is any indication then the Clan stuff is off the roof in terms of pricing. And with the way they are selling it, everyone and their mom have clan tech. I don't know if you can mount them on IS mech yet, but I cam smell the min/max cheese if they do. I know a certain someone at least would complain about it, but I am sure we find OP ways to build lots of nasty creations.