Culligan (?) told you that you CAN"T hook up a softener until the AC filter is in. I don't see why that would be a requirement. It is recommended, yes, but not required.
What is your hardness? Any iron? Other test results? The MidAtlantic system: does that include one or two upflow tanks? At that price, my guess would be one. If installed, and depending on the media actually applied, that would be a great price. But as with all great (read: CHEAP) prices, there are many times serious drawbacks or shortcomings, either immediately effects or in future service and repair/replacement costs.
The low pH will have little effect on the softener either to its benefit or detriment. However, once that AC filter is installed, a slight rise in hardness will occur and the softener will need some adjustment. I am guessing your source water hardness is between 2-6 grains per gallon. Be sure to check carefully for iron.