Had a pup who was born with 6 toes instead of 4 on 1 back foot, causes and solutions? thanks

If I'm seeing your picture correctly, those would be on the inside of the foot?  And since I knew nothing about polydactyly in anything other than the occasional cat, I had to go looking.

Personally, if they're not a problem to the pup, I'd be inclined to leave them alone.  From reading, which I presume you have done as well, removal when the pup is neutered would be the obvious time.

I have one dog whose dewclaws are the loose and floppy type (attached by skin only, no bone connection) and they bother her not at all.  I've often wondered why they don't get caught in the brush around my place but they apparently don't.

However, there was one article on a Great Dane Forum and you might find the last entry of interest.

If he gets around fine and the toe is well formed enough that the nail isn't trying to become ingrown or infected then I wouldn't attempt to have it removed. A lot depends on the extent of its formation. Siberians should not have extra toes so I would consider this a birth defect likely from inbreeding. Definitely get him neutered and do not use him as breeding stock.