It's possible to have completely different Path Selection Policies installed within your ESX environment, as typically rules are set by each vendor's plugin on install to select the right PSP when the volumes are mounted. You can check this by looking at an EQL volume (should have the EQL PSP auto-selected) and a Nimble volume (which should say Nimble_PSP_Redirected).
The only time i've ever seen problems with EQL & us co-habiting host side software is in Windows when both Windows Toolkits are installed, and that's mostly because the VSS requestors get in the way of eachother.
Hope this helps! Let us know how you get on.