Can you say неправильный instead of неверный?
Yes you can. It is accepted now.
username — имя пользователя not логин!!!!
If the two words had different genders, which genders the adjective be?
With или, the gender will match the first word and the adjective will be in the singular (if the first noun is in the singular, of course).
Do russian people have original Russian words for "login", "site", "flashdisc" and other modern words, especialy words used in this course ?