Having taken probably hundreds of taxis in Sweden and lived and travelled there many times, I would say that where you sit should not be an issue per se. But, as with anywhere in the world the driver needs to feel comfortable and you might be unlucky to find an awkward one.
If you are unsure then you can also say to them the reason why; most will not have a problem with it. They will soon move their bag, the same as they would if 3 or 4 passengers got into the taxi.
Also, it is worth noting that in Sweden you are not obliged to take the first taxi which is in a taxi rank and you can go down the rank to find one which is more likely to accommodate your request (or is cheaper!).
You also refer to London black cabs not having a front passenger seat. In this case you would need to use a mini-cab company, which you would have to call and hire or use a service like Uber instead.