This thread went from a new fish in a tank, to adding more fish. If you want to add more fish there are a lot that will be fine with a Sev in a 55. If the only reason you want more fish is to what amounts to pestering (dithers) the Sev to come out so you can see it, I would suggest you let the Sev settle in and it will come out on its one. I have had more than a few Sevs and they are fairly active fish that are not prone to hiding.
If a full tank of fish is what you really want, there are other options than just tetra.