FediTips has moved!Ice Cubes, Mona and the official app are free.
If it said the username is taken, it sounds like you might have clicked on the option to sign up for a new account, rather than sign in with your existing account? Normally both options are shown when you first use the app.
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.