..and hi to you!
#1. Can I put a 100 in screen for an 18 ft distance? or is it too big?
Typical screen size is about 10" to 12" of diagonal for each foot of viewing distance, with people tending to go larger (12") per foot of viewing distance. Screen size should be determined by viewing distance or space constraints. So, 100" is small (IMO), but with your limited ceiling height, you will have to roll with what works. I have links below to help you visualize your setup!
#2. Which screen would you recommend?
I would strongly recommend the Silver Ticket fixed frame screens in white, and I would strongly recommend a good coat of dark paint on the walls and ceiling to help make the image look really good. Maybe some dark carpet if you can.
#3. Given my ceiling height, can I install dolby atmos or is it not recommended?
I'm going to defer to others on this. In theory, most people think Atmos sounds good. If you are running cabling yourself, or need cabling run, then adding it isn't that much money. Don't go crazy on these effects speakers in terms of cost.
#4. Which speakers would you recommend?
You will have a lot of shopping to do here and a lot of research. Decent speakers and a very good subwoofer would always be the way to go. But, there are a 1,000 good models.
Video and audio are very different beasts, so I will stick with the projection answers.
For a projector, you will need one with lens shift so you can place it at the proper height and get the screen hit. You will want to consider the screen size and the lens to screen distance. If your projector is 18' wall to wall, then it's about 16.5' lens to screen. That's the throw distance. So, you need to use a calculator to determine what screen size you can hit from that distance. Projector Central has a good (the best) database to just search by.
Going up to 120" diagonal instead of 100", we get this as our search result with a 16.5' lens to screen throw distance and a price point under $3,000.
https://www.projectorcentral.com/projectors.cfm?g=1&hide=0&st=1&r=&br=&c=&w=&ar=16:9&db=&zr=&wt=<g=&ll=&mfg=&p=800&p=3000&wr=&dt=&t=&exp=1&hls=1&pjl=0&pjw=0&pjh=0&td=16.5&is=120&i=d&tr=&tr2=&oop=1&sort=pop&sz=15
On that list, the Epson 5040 or the Sony HW45ES may be the best looking models. I would also look for a JVC RS440/X590 projector as an option if you can find it at a good price. It will look better than anything on the list.
You can modify the search parameters to add/remove items, and if you have a question about a specific model, just ask. I know all the answers (ha ha).
Now, for the best projection calculator on the face of the earth...
http://www.reviewtranslations.com/projection_calculator_en.html
Select a make and model of projector, set your room size (or your theater space size) and drag and drop and move things around. Play with it! It's not perfect, but it works very well and can give you a very good idea of what placement will look like in your room.