The rate of growth kinda depends on gender and genetics but under good conditions they grow rather quickly. At about 5 inches I would say 2-3 months old but it's only a guess. I would try to switch him out of the 20 gal once he reaches about 12 inches. The dimensions of the future tank are great and the iguana world setup is actually quite good but I'm sure you would want to scale it down. Only thing I would kinda change or add is make sure there are plenty of vertical climbing spots in there.

One of the most popular dewormers is Panacur which could be bought in a number of places if it isn't available at your job. I do recommend a fecal before treatment and keeping the sides of the enclosure covered for a week or two and reducing the handling.

As that small of a baby I usually recommend a paper towel substrate or reptile carpet because it can be more difficult for younger reptiles to pass things (especially substrates) but the choice is yours. I would recommend you wait until he has settled in for a week or two before you go changing things though. If you visit the link in my signature there is an article on humidity and how to keep it up as well as a more active forum than this one.

I use the same supplements as you do but my scheduling is a bit different. Multivitamins aren't as needed as calcium so it can be offered once or twice a week and calcium should be 6 days a week for dragons under a year/year and a half. When you supplement you only need to dust one feeding a day. Getting them to eat plant matter can be difficult. They are insectivorous by nature but may snack on greens. It isn't a do or die thing with getting them to eat greens.
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