Yeah you really need at least a liter of water with it. Seriously. Malto and dex are not all that sweet by themselves so you thinking it's so sweet makes me think it's very concentrated which is not a good thing especially in a pre shake. Big plastic slurpy cups do good. Or jumbos from McD's....
. You can also get a shaker to mix shakes in but they tend to only go to around 700 ml's. Still that's better than an 8oz glass or something.
I personally don't like stiff-legged deads. I prefer either Romanians or pull-throughs as far as that kind of thing goes although there is a huge amount of variation in the way people compare the two. Since stiff-legged's keep the weight out further away from you center of mass it does tend to put more stress on the back like you say. Course the question is whether it's bad stress, lol. IMO anything that puts the weight out front in an artificial position like that is not all that good for your back especially if it invovles rounding out the lower back in order to give it more work. It prevents you from effectively transferring shear stresses into compressive stress. Same thing with good-mornings. I do tons and tons of posterior chain work but stiff-legged's are not in there. Personally I've never found that may lower back needs "extra work" of that kind.
The other problem is how much you bend or don't bend the knees. There is not ever legitimate reason to not bend the knees to some extent. The goal should be to work the entire posterior chain and all the extensors. Not to try and bypass the glutes and only hit the hams and lumbars. That's stupidity in motion. Most people don't get their glutes involved enough anyway. And for the hams to be fully recruited you still need some knee movement,, let alone the glutes....so call it SEMI stiff-legged deadlfits which would mean that the lower back stays set (in a natural arch), the knees are bent but the the butt doesn't go back as far as in Romanians so the weight is a little mroe away from the legs.
I think a lot of people are really doing something more like Romanians anyway but they call it SLDL. Ask five people about them and you'll get 5 different explanations.
Personally as far as semi stiff legged variations go I stick with Romanians because I can use tons of weight and not have to go shit light so I don't injur may back.