Newbie here! Was just wondering how you merge different brushes into a single brush(?) or model or what it's called? I've tried using CSG Merge but all it says is "CSG Merge: Failed - result would not be convex."
Brushes must be rectangular boxes, or anyway n-sided closed polygons (like cylinders or D&D Dice), where nothing is sticking out. That's what the "convex" part means. All the angles are "inward" (less than 180 deg measuring the inner side) and an angle "outwards" (more than 180 deg) is forbidden. So if there's any part of it that looks like an "L" shape, with some part sticking out, that's concave and it can't be a brush. You have to use two brushes for that, one for the | part and one for the _ part. Long story short, if you can merge two brushes, you can just make one brush the same size & shape as the merged one, so just make that.*
It depends on why you want to do this, but one thing you might be thinking about is linking brushes together with the "group selection", then you can easily select them simultaneously and work with them as a group, or something like that. That may be what you're actually after.
*Edit: You can imagine cases where you really need brush merge to make a shape, like the 8 sided D&D dice. You can make two 5-sided pyramids/prisms (4 triangle sides & a square base), and then stick the bases together top-to-bottom and merge them to make the 8-sided dice shape. It's convex so still a valid brush, but you couldn't make it naturally as a single brush any other way (I think). That's an example of what it's really for, which is really a kind of special case.