Do you foresee any problems?
How can I change the ISPc3 default port...? (Search the forums for a start)
The only problem is the port number.
You can either change the port number of tomcat in somewhere/tomcat/conf/server.conf or change the port of ISPconfig in somewhere/httpd/conf/sites-available/ispconfig.vhost and restart apache. (I don't know if is configured somewhere else also). An update of ISPconfig also asks on what port to run ISPconfig.
Afterwards you may want to install proxy_ajp to setup apache as the webserver for tomcat.