Trying to keep my install as clean as possible and avoid top nailing, however since laying an underlayment I don't see anyway around it. The underlayment is not glue to sub floor.

If I do have to top nail, which nails are recommended? Are 18 gauge brad nails sufficient?

Thanks

if its just felt just cut it back and use some glue on the last few rows thin pin nail along the wall that should eliminate the majority.