Unfortunately, I can attest to the fact that Culligan tanks use a different thread than "regular" tanks. I had a Culligan "Fleck" 7000 1 1/4" valve system on a 12x65 Total Home system with 2 cuft resin and 1 cuft carbon. (They told me it is the valve unit used for the Platinum Plus series of softeners.) Long story short, Culligan and I had a severe disagreement and legal frivolity ensued. After resolving that difference of opinion, I replaced my entire system with separate softener and carbon tanks, using a Fleck 7000SXT for the carbon and Fleck 7000XTR for the softener.
I had attempted to reuse my Culligan valve but my new water guy insisted on using a new tank because he said the labor to clean and refill the old Culligan one would have eaten up any savings. The price was right so I didn't argue. Turns out that the down tube which the Culligan valve uses is a different size and didn't mate up with the one he'd ordered for the new tank. He said he couldn't get the old Culligan tube out so the tank got chewed up getting it out. (Unfortunately, he didn't figure out that the top basket just unscrews and then the tube comes right out. ARGH!)
My Culligan Fleck valve does not screw onto the top of the tank. Instead, a threaded insert screws into the top of the tank and then the valve sits on that with an o-ring. The insert and the valve then have a collar that clamps around them to hold them together.
Unfortunately, it was only at this point that my independent water guy goes to put the old valve on and discovers that the Culligan insert has different threads and won't screw into the "industry standard" tank. He said that he contacted his wholesalers and that no one made an insert with industry standard threads to use with the Culligan valve. That required me to spend money on a new valve. Frustrating, but not as bad as it could have been because I got a good price on the new valve, a new warranty, and the availability of replacement parts.
Side Note:
I do have a beautiful Culligan Fleck 7000 1 1/4" valve sitting in my garage. It was installed in 2008, sat unused on plumbing bypass for a while, and then was recently tested four times a few months ago. If someone ever needs one but doesn't want to deal with the C-word people, I could be convinced easily to part with it for a reasonable cost.
I would NEVER do business with the C-People again.