Can't install drush8, having drush6
Can always install manually

# get drush latest 8 version from 
# https://github.com/drush-ops/drush/releases
# or any version that you want 
wget https://github.com/drush-ops/drush/releases/download/8.1.16/drush.phar

# make it executable
chmod +x drush.phar

# move it to home directory or any other directory
mv drush.phar ~/drush

# go to that directory
cd ~

# find out the path to the directory
pwd
# the output would look similar to this
# /home/your_user

# edit the following file 
nano .bash_profile
# or in some systems
# nano .bashrc

# add this the end of the file
alias drush='/home/your_user/drush'

# use the alias you just setup
source ~/.bash_profile


NOTES: 
Above example works in ubuntu but should be similar on other distros

According to the docs drush 8 works with D8, D7 and D6 so no reason to install drush 7

If you want to call it with drush8 use following alias drush8='/home/your_user/drush' in .bash_profile file