You could make a command alias.
alias scp="scp -l 100"
Different distributions handle this in different ways. My PCLOS system has a script /etc/profile.d/alias.sh which contains commands that are executed when anyone logs in. Aliases are defined like that. Other methods used on UNIX and Linux systems include putting log in commands in /etc/bash.local or in each person's $HOME/.bashrc or $HOME/.alias.