Or am i right in thinking you can double tag all lambs with full EID and a management tag and that's still ok?
Yes, that's absolutely fine, and is what most smallholders do.
As a general principle, the bigger the tag, the easier it is to read (duh...), but also the easier it is for it to catch on something and pull out (you know, I sometimes think my vast intellect is wasted on farming
).
Almost all of the sheep we bought in came with button tags, and almost all of them have lost them one way or another. We replaced them all with Shearwell "SET" tags, and have only lost one so far. The disadvantage of the SET tags is that the numbers are too small to read from a distance.
The colours on the visual tags don't mean anything (except red, which is reserved for replacement tags), so you can do what you like - lots of people change the colour each year. Our sheep are all pedigree, so have an additional number and year letter on the tag. What we do these days is to use any left over tags from last year to tag this year's wether lambs or others destined for slaughter. They get tagged when we ring their balls, and the colour makes it nice and easy to identify them at weaning time.