I don't think that you are are overthinking this. How you are perceived by a prospective employer comes across in every word you give them. Though this is true for emails and digital communication throughout your career, once you exist in an organization emails to your boss and coworkers can be more organic.
Your reply does everything that an email should do: you are acknowledging the information given and providing exactly what was requested of you, no more, no less. It would be rude to reply either: "Skype ID: XX", or a lengthy email outlining points that will be discussed on the call.
How can it be improved? Because these few words are representing you, you want the most bang for your buck. You are wanting to say "This information pleases me", without sounding like a cartoon villain.
Personally, My reply would be:
"Looking forward to our call, [employer first name].
My skype ID is XXX.
With this, you've acknowledged both of their sentences and addressed the email to the recipient. It's clean and simple without puffery; formally informal.
Addressing emails to their attending recipients is very important, especially if that email has a chance of being forwarded or has multiple CC's. If I am sending an email to 3 people, I'll name all 3 of them: "John, Mark, Frank, ..."
Saying "sounds great" may give the impression that you are reserved in your enthusiasm. It doesn't just sound great, it is great. The call is going to happen, it's not conceptual. Try not to say words like sounds, feels, seems,etc. You want to be definitive.
Saying "great" on its own has risk of sounding sarcastic IMO, and exclamation points should never be used in emails. Replace "great" with something like delightful, excellent, marvelous, wonderful, fantastic, fabulous, splendid, etc.
Also, you know exactly when the call is. Replace the vague "talk to you soon" with the definitive "talk to you then". This affirms you understand there is a call and that is will be happening at X time.
I often put 'Thanks' (for more informal emails) or 'Kind regards' (for the more official ones). Most emails don't need anything!
This is an interesting article on the subject of closings: http://www.bloomberg.com/news/articles/2015-06-02/the-best-e-mail-signature-is-actually-the-worst
This gets the job done too:
"Splendid, [Emp. first name].
My skype ID is XXX.
Talk to you then.
The little things can have as big an impression as the big things.