Mixing Tiger Barb Varieties

The barbs themselves should mix together fine. They are the same species and that seems to be all that matters to them. You will have squabbling when new ones are introduced, whatever the colour, as a new pecking order will have to be sorted out. And the bigger the shoal, the less likely they are to pick on any other fish in the tank. I've seen 10 recommended as the minimum for tiger barbs to keep their aggression within the group.

In theory, a mixed group could cause problems with breeding as the resulting fry will be a mix if the parents are of different types. But it is far more likely that any eggs would be eaten before they get as far as hatching, and even if any survived long enough to hatch, the fry wouldn't last long. For deliberate breeding, a tank with something on the bottom, eg mesh, marbles or grass matting, to protect the eggs is recommended.

Green tigers are amazing fish and are on my want list, mind you that would mean getting yet another tank and I kind of think that 5 is more than enough, Maybe....

I have kept a mixture of Standard, Green and Golden Tigers together in the same tank with no great impact.
As Sue stated, they squabble at first then settle down.
In my experience, the Golden Tigers appear to be the least aggressive of the three types.