It may be possible in Unix, if you install each version on different directory (this mean 2 engines will use different onconfig and sqlhosts fiels), use separate chunks, use different environment variables for informix (this mean you should use different user to start informix).
It is impossible on Windows NT because sqlhosts file is in the same registry in the same machine.
Definitely on Unix. However, I would exercise extreme caution as multiple instances will degrade performance based on the usage and size of the instances as well as the hardware on the server (disk, cpu, memory...). Also performance troubleshooting is much more difficult.
I do not know your environment but a question I have is what is preventing you from just upgrading your 7.x to 9.30 ?