Mac newbie, advice request for new iMac setup

Hi all. Mac newbie here, but experienced pc'er. I am about to convert my household from PC to Mac. Just received a new 21.5" iMac for the family (family room for kids and wife) with a 27" soon to come for my office. I am a few screens in on the setup and was hoping for some advice. Here are the particulars:
- We have two iPads (my wife and I) and 3 iPhones (my wife, my daughter and I) in the family.
- my mac id handles all itunes store purchases. It is setup on all devices for store.
- I set my wife up with what amounts to a sub account - her own icloud id that is used for pretty much everything except the store.
- same setup for my daughter.
- setting up the new family room iMac. I'd like to set her up as an admin and each of my two kids (one with the iPhone) as users with parental controls.
- I would like to be an admin user, although I am more likely to use the one I will be putting in my office.
- I recently picked up a Synology unit which I would like to become centralized storage for all media as well as a digital filing cabinet (as I convert paper)

Question(s)
1. as I was setting up the iMac - it requested my AppleID for store etc. I gave it mine - the store id for the house. If then asked for setup for icloud and offered me all of the options that are associated with my store id - which is my id. What should I do from here? My wife will be the primary adult user of this iMac but it is only associating with me, because of the Apple id, at present.
2. Will I be able to use messaging only when signed in as myself on this iMac? I'd like to be able to use it, but not at the price of my messages showing up for all users.
3. Is there anything else I would need to think of with regard to the external storage ... which has not yet been setup?
4. My wife's iPhone and iPad are getting low on storage, mostly because of taken photos and video. Can I use Photostream to move it to the iMac (and then to the external) so that I may delete them from the iPhone and iMac? I thought I had read that deleting from Photostream may delete in all places. I would like to make sure this doesn't happen.

Any other thoughts and advice for someone new to a Mac world would be appreciated as well.

 

So should I be the first user set up? Because of the AppleId (which is also my iCloud id) - it is seeing only me for now. Would I select whatever applies to me and then set the others up? Doesn't seem that I even have the option of setting anyone else up at this time.
Not really hiding anything . Just think it may not be a good idea for my kids to see what may come across from one of my unpredictable buddies.
My wife is to be the primary user. She has her own iCloud id. The thing is - the setup screens do not ask for the iCloud id, only the AppleID (which is mine).