Yes, you can use an sd card as internal memory in Marshmallow. I saw somewhere that MM is coming to Sony devices from March, but I'm not sure which ones will get it and in what order.
8gb is no longer really a practical amout of memory. Although the OS take up about the same space, it does creep and, more to the point, apps are getting bigger as they offer more and more content and facilities. As bandofbrothers says, 16gb is reckoned to be barely adequate for an iPhone.
All that said, I had a Vodafone Smart 4 (8gb) for a while last year and managed to pack quite a lot onto it. It was already incorporating the (small) sd card into its memory pool, so was using it automatically for app data. I also kept photos and other files on it, but didn't transfer any of 20 or so apps.
The other issue you might have is finding enough space for the OS upgrade if/when it comes. If you're really not happy, I'd be inclined to swap it for something else if you can. 16gb of storage is desirable (with an sd card slot) and more than 1gb of RAM, too, though that tends to be the preserve of considerably more expensive devices.
