Q: Should I glue the wood planks together, as I nail the wood floor down? If it is a bad idea, what could happen if I do?
A: I don’t think it is a bad idea. Maybe not necessary. What type of adhesive are you using? How wide are the boards?
The biggest issue with a wood floor is moisture control. Control that and you won’t have any problem with expansion.
Otherwise, if the sub floor you are installing to is not the best and you want adhesive for better hold, go for it. Probably a polyurethane adhesive is best.