Good observation on the difference between steeping and mini-mashing. If you're steeping, all you're doing is dissolving sugars and rinsing them from the grain. Anything that has to do with mash parameters (including water profile) isn't involved since you aren't mashing. You can still pull tannins from the grain if you go above pH 6 and temperature 170 simultaneously, however.
You can use the same amount of grain as long as you are getting it all wet and allowing the water to freely rinse it. Typically, you're going to be fine unless you have a very large amount of grain. When I steeped, I used the full volume of water that I'd use in the boil. Steep, rinse, add extract, boil, chill, ferment.
I wouldn't use base malts or starchy adjuncts unless you're going to mini-mash. You'll get the flavor from the grain but you'll also pull starch from them. Not good for clarity or stability, although much of it will likely wind up as break.
If you're going to mini-mash, pay attention to what you normally do during the mash (grist to water ratio, water profile, pH, etc.). But if you're going to go to all that trouble, I'd just as soon do it as a full all-grain batch and be done with it.
If you're steeping, water profile is irrelevant except for flavor.