I've got the SW bug and multiple tank syndrom,
It's ok, you're amongst friends here
There isn't really a size limitation when it comes to running a skimmer and or a sump. Somewhere around 45 to 75 gallons is a good size for a tank like this since you have enough water to help buffer against big swings but the tank isn't so large that the costs and work involved to maintain it doesn't turn someone off the hobby. It all depends on what you want to accomplish and what you are willing to do to maintain a tank. Obviously sumps/refugiums and skimmers are going to provide
added insurance and benefits in terms of stability and nutrient reduction but as long as you realize the limitations of running a tank without them, you should have success.
Topless systems are popular because of salt creep for the most part. Heat buildup (less of an issue with LEDs) and reduced gas exchange are other reasons you see so many topless tanks. Salt creep is not only unsightly but it builds up quick and blocks light. Mesh tops are pretty popular as they keep most fish in but also do not have the issues that glass/acrylic lids have.
Most of the dwarf and pygmy angels are listed as "with caution" for addition to reef tanks. Larger angels are pretty much a defininite no (although I have seen a few). Some dwarfs are regarded as more reef safe that others. It's really hit or miss with each fish. Some have flames, potter's, coral beauties for years without an issue and then all the sudden they go on a rampage.