Are there any configuration or installation hazards to setup both Postgresql-8.4 and MySQL on the same server and running them side by side, other than performance?
The server is running Debian etch.
i do have similar configuration live. they [ mysql-server-5.0 and postgresql-8.3 [ not 8.4 as you ask ]] work fine together.
it's not like with smtp servers postfix vs exim - mysql and postgres do not replace each other from the system perspective. but obviously those two db engines will compete for system resources [ cpu, io ] if you are going to put high load on them [ but there is nothing wrong with it ].
No should run OK - and you can even minimise performance issues if you have their data on separate physical drives and enough RAM/cores.