Upsetting the pack balance? please help!

Hey if anyone can offer their opinion on this issue I'd be hugely grateful as it's causing me lots of worry

I live in a household of five dogs (two of which are mine) so am very accustomed to living with and dealing with a 'pack'! Three bitches and two dogs, and they all get on well together and we have no dominance issues from them as all humans in the house are at the top of the pack if you like. We're careful to ensure their care is all kept even between them in order that we don't create issues between the dogs for dominance etc.

My partner has one dog (a spayed bitch) who is a sweet dog (a little ansty with dogs she doesn't know but that is not the concern). My partner adores her dog and (by her own admission) is not hugely keen on other dogs and has never lived with or dealt with a pack of dogs before. My dogs and her dog have yet to meet and whilst I do anticipate the likelihood of a fair stand off whilst they sort themselves out, my big cause of worry is travelling them.

My partner does not drive whereas I have a transit van, and when I take my two (or the five) out they all travel in the back together. My partner is utterly adamant that her dog cannot travel in the back of a van. In essence, should she and I be travelling somewhere together with just one of the dogs, I do not have any issue with the single dog travelling on the front seat with us. However, my feeling is that once the dogs have established a pack order amongst themselves, then if we are travelling with all of them they should all travel in the back of the van, as by having one dog sit Up Front with us we would be causing imbalances between them which we really don't need to do (I'm talking once they know one another, not suggesting just stuffing one poor dog into the back with the others and letting them get on with it!).

It sounds so, so silly written down, but it's causing me worry as I'm very anxious not to upset the hierarchy between the dogs and I know from experience that sometimes singling one dog out to do something very different from the others does not lead to happy times. I do not understand my partners issue with her dog being in the back, but she would rather the dog travel in the front and sit in the back herself than let the dog be in the back of the van. I've heard of dogs getting the way between people before, but never experienced it before and it's not a whole lot of fun

I'd be really grateful for peoples' opinions on helping keep the dog hierarchy even and if people think something like one dog in the front whilst the others are in the back would be an issue in upsetting this? Am I wrong to think this might cause an issue? Any suggestions to help keep the pack order settled which us humans can employ would be well received too.

Many thanks, and apologies for the long post.