One rough but really quite good guideline:
What is your likely “healthy weight” in pounds? Suppose it’s 160 pounds.
While on a severe diet, you should probably consume about 160 grams per day of protein. The rest of your calories can come from fats or carbs, assuming you get your necessary vitamins/minerals/essential fats.
That may seem like a very high level of protein, but there’s a lot of support for it during very low calorie diets; you can search on PSMF (protein-sparing modified fast) if you want to see some of the literature.
Your 1000 calories of Soylent are only providing 40 grams of protein per day, a very low amount while on such a severe calorie deficit, so you could supplement with an extra 120 grams, to bring it up to 160 grams of protein. That will bring your total calories up by 120 x 4 = 480 calories, for a total of 1480 calories per day. This is still a sharp calorie deficit at your size, but the boost in protein will do much to preserve muscle mass and other lean body tissue. At this low calorie intake, eating less protein may lead to faster weight loss, but the additional amount lost will be largely lean tissue, not additional fat. You can adjust the numbers for your real “healthy weight…” if you stick with 1000 calories a day of soylent, your protein target will be (bodyweight - 40).
As far as brands - I used to use Optimum Nutrition, but their prices went up. Currently, I’m using:
It comes in chocolate and vanilla, and flavors my soylent nicely. It has a small amount of fat and carbs - 2 or 3 grams of each per 23 grams of protein. You can mix them with water when not drinking Soylent, or add them to your meals.
I also use http://www.amazon.com/gp/product/B0015AQL1Q?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s00 to make different mixes and flavors - it’s a neutral, flavorless version. It’s more expensive per bottle, but comes with more actual grams of protein per bottle - so it doesn’t really cost more. There’s almost zero fat or carbs, and it tastes like it. It’s OK in Soylent, or when flavored, but terrible alone. I mean, terrible. Not a good late night snack.
It doesn’t make a lot of difference when you consume it - there are myths about not being able to take in more than 30 grams of protein per meal, but those don’t hold up when tested. The better strategy is to take the protein in whatever way you tolerate best - you may find a small chocolate drink a great way to stave off late night hunger, for example… or you may find a high-protein addition to your regular Soylent meal leaves you satiated longer. Experiment and do whatever works best for you, as long as you hit the daily targets…