No Maemo OS is not compatible with E90 communicator

No you cannot flash your E90 with maemo OS so its 100% working fine. I don't think you would even be able to flash your E90 with maemo firmware because its compiled differently. Even if you forcefully flash your E90 with maemo OS you'll end up bricking your E90.

Here is a brief comparison of the difference between N900 and E90. The main drawback is the CPU in E90 along with the internal capacity. N900's firmware requires 256MB oneNAND space which E90 does not even have, and that is just for the base filesystem.

Apart from comparing with just N900, E90's specs are somewhat a little similar to N900's predecessors such as N8x0 and even 770 however you still won't be able to boot a completely different bootloader + kernel due to signed bootloader. So you're still stuck either way.