All BT powerline adaptors/extenders are compatible with each other (as they use the same Standard). So you should be able to successfully have the BB Extender 500 [BBE500] and Mini Connector [MC] in the same setup.
We recommend ensuring that the Mini Connector is connected as the primary extender in the setup (so the Ethernet from the Router/Hub is connected to the Mini Connector).
To link the BBE500 to the MC please do the following:
For ease, it's best to complete the pairing in the same location as the primary MC (this is so you can easily observe the LED indicators). Once paired the BBE500s can be relocated to where required.
On the BBE500 (do one at a time) press and hold the 'Link' button for 5-8 seconds
Wait for the 'Power' LED to return to solid green
On the BBE500 press the 'Link' button again for 1 second this time
On the MC (within 2 minutes) press the 'Add' button for 1 second
Wait until the 'Power' and 'Data' LEDs on both the BBE500 and MC turn solid (green on BBE500, blue on MC)
Repeat steps 2-6 for the other BBE500 extenders you wish to add to the network
Relocate the Extenders around the house - they should automatically reconnect when plugged back in
Hope this helps, just give us a shout if you need any further help.