My first few tanks I skipped the sump as well. I was new, I didn't quite get it and I was scared. lol
I have a 55g now that I am running a fluval canister on with no ill effects. For a 55g I would suggest having at least 60-70lbs of rock (you can get mostly base or dead rock and the rest can be "live") and running a hang on back filter. The rock is about 80% of your biological filtration and the hang on filter is just for mechanical filtration pretty much.
If you are starting fish only with live rock then I think you can wait on a skimmer, you'll just have to be religious with your weekly water changes and not overstock and overfeed. The skimmer removes organic material from your tank so the less you have the better.