Shimano LX 581 (2007) with 7 speed?

Can I use a Shimano LX rear mech

http://www.merlincycles.co.uk/?fn=produ ... egoryId=76

this one with a 7 speed MTB set up?

Never used a 9 speed rear mech for anything other than 9 speed

i know road mechs can be interchanged eg 10 speed dura ace works with 8 speed etc..

but can MTB mechs be interchanged?

thanks