I too am having to make a decision about getting my 13 month old min schnauzer neutered.  I consulted a behaviourist because our 4 year old wire fox terrier has stared attacking it.  He does not bite him but turns on him vocally and also pushes him down most times without cause.  The behaviourist advises that it is because the min schnauzer is giving off 50% more testosterone and the wire does not know where he is at.  I find this a bit strange as most of the time they are ok together, I can leave them all night together and nothing happens.  I also can leave them together during the day if I have to go out and they are ok.

Any advice as I really do not know what to do.

It's difficult to advise on a situation like this without observing the dogs involved.  At 13 months your Schnauzer is a teenager, and yes, his hormones very probably are all over the place.  But it's interesting that these 'attacks' only seem to happen when you are around - could it be that the older dog looks on you as *his* resource, just as another dog might look on toys or food as a resourse, and that the Schnauzer is intruding into that?  It's also interesting that the Fox Terrier isn't biting, this is VERY restrained of him, and he must feel he's getting his message over without having to really lay down the law.  What do you do when they kick off?