I admire you for picking up a lost soul. I wonder if you are having any behavioral issues that usually come with re-homed iguanas. I rescued one of my first iguanas from a pet store, too, and he was as strong and mean as a demon. In two years he had turned into a pussycat.
About what I write, scrutinize it and build a wider base of assent, because I'm not perfect. But I'm confident about the stuff I've written to you below.
1) Are Zoo Med 5.0 better than Exo Terra 10.0?
Well we have no comparison anymore. About ten or 15 years ago someone collected all the various UVB emitters and measured them with a UVB meter, and created a table of relative emittance for us. Even then, we had no way to compare one of them to an "absolute value" because we'd need a physics lab to have the equipment to get a number.
For two or three decades, the UVB 5.0 was the only choice, and it finally enabled us to keep or igs healthy. Since then there is the REptisun 10.0, and several mercury-vapor lamps that give more UVB than the 5.0 and are very popular. Still, the 5.0 is perfectly adequate if it's hung down to 6 or 7 inches from the top of the iggy's back. If I had the money, I'd opt for the fancier, higher output, and more expensive UVB lamps. But I would be careful how I decide to choose an exposure distance from the iggy's back.
2. But I would like to ask: Should I measure the distance from basking floor to the tubes or from the iggy's back
For the ReptiSun 5.0, six to eight inches is the only rule I've ever seen (except that I always hung the 5.0's at the unpopular distance of exactly six inches, and that worked well for 14 years. I hung the 10.0's at 8 to 9 inches, as a pure guess, which worked for my iguanas.
"These summer month I had given him only little vitamins or calcium. But if you say - no - I won't add them to his food any more."
Remember the dose I specified? I was serious about that. If you are more comfortable when you supplement, use exceedingly small doses of a supplement without any of the Vit. D's or iron. (A bit the size of 1/4 of an aspirin pill should be ok.)
Malnourished iguanas, and those fasting for more than 4 weeks need supplements. Deformed iguanas need a prescription calcium supplement until it's blood levels are right again. It's a controvercial issue. I come down on the side of feeding high nutrient greens soaked in water. With iguanas, it turns out that water is the most important supplement of all. Use any trick to get more into him.
"cabbage, carrots, spinach, bok choy,". All can be fed in small amounts continuously. Just keep them under 15% of the total eaten diet. Romaine Lettuce can be fed freely when fresh leafies are hard to find.
About what I write, scrutinize it, because I'm not perfect.
Do you happen to know that frozen greens retain all their nutrients except thiamine? Iguana keepers all across Canada feed frozen greens during the winter, and sparingly supplement with thiamine. I used to feed frozen-thawed greens once in a while too.