If the tank is completely cycled, you can start by adding a small fish or two. Never add a bunch of fish to a new tank. This usually ends badly. Tend to the fish (or two) for about a month. If everything goes well, you can add another small fish or two. Do the same thing... tend to them for a month. If all goes well... you get the picture.
You don't need a skimmer right now, as there won't be much for it to do. Once the tank becomes more stocked, you should consider getting one, as they remove a lot of nasty stuff from the water.
If I have to choose between a sump and a refugium, I will always choose a refugium. A refugium will hold a skimmer and return pump just like a sump, but it also has a separate chamber for macro algae to grow. Macro algae helps in removing ammonia, nitrite, nitrate, and phosphate from the water, which can greatly reduce the frequency of water changes. It's also a refuge where pods can live and breed. Some of the pods will work their way into the tank, where they become treats for many fish. Free food is a bonus to having a refugium.
If your T5 lights are bright enough to light the tank, they should be all you need for a fish-only tank. You didn't mention how many bulbs you have, so I don't know if it will be enough for corals. Example: Two bulbs... fish and low-light soft corals. Four bulbs... most soft corals and low-to-medium light LPS.