A randals goby would be fine, so long as you have a top. Watch putting too many bottom gobys together. I have heard twinspots are not very hardy, but do better in pairs. It can be hard to get them to eat prepared foods. We have a tailspot blenny in our 20g and he is a cool little guy with a great personality. I would go easy on stocking, a little at a time. After 5 weeks, have you totally cycled?