I'd recommend moving the regal tang to a bigger tank once he hits 2-3 inches, which might take 6 months - 1 year. A 75 gallon (minimum) would be ok for one regal, or sell it if you arn't able to upgrade.

I wouldn't worry about moving the engineer goby, they have relatively low metabolism and often are content to sit in a cave most of the time as adults.

I also advise against adding any other fish to this size tank - you're pretty heavily stocked for a 29, even with a sump.

Best of luck,

andrew