I write with two spaces after full stops. That's how I was taught back in typing class 25 years ago. My wife writes with one space after it. (I believe that comes from writing press releases for her company.) It wreaks habit when we try to co-author something. I should probably try and convert myself to a one space guy, but it would be towards the bottom of my list of things to work on in my writing
On leaving out the comma for combining two short sentences, I believe it is a matter of style. You don't have to exclude the comma, but it is fine if you do. I've never heard of a publisher having a preference. In the long run, it's pretty trivial.