I'll answer half your question (all channel partners in one project). I'm still working out the best answer to the other half myself (all documents in the set in one project).
Definitely, definitely put the documentation for all the channel partners in the same project. That's exactly what I do, and exactly what Flare is good at. To keep it simple for explanation, let's consider a single document that has a PDF output, produced for each of your four brandings (I have three different brandings).
I have the following:
Variables for things that are different across brandings (in my case, for example, the product name, release date, software version etc)
Conditions for each branding, to allow me to control minor content differences (in my case, brand A, brand B, brand C, so I have
A Only, B Only, C Only)
Different page layouts for each branding, again in my case, one for A, B and C.
Some different topics, for example the Title page and Preface section for B and C are specific to the requirements of our partners.
Different styles for each branding. You can put them all in one style sheet and use different media, or have separate stylesheets, which is my personal preference. For example, my heading 1 style for A is coloured, and black for B and C. Fonts, spacing, table styles etc are different too.
Now it gets clever . . .
A single TOC for the document, that contains all the topics for all the brandings. For example, it has all the title pages, all the prefaces, all the topics. Where a topic or part of a topic is not relevant to all brandings, it has a conditional tag. For example, topic X only applies to A and topic Y applies to A and B. So X has condition A Only applied, and Y has condition A Only, and also condition B Only.
A target for each brand of the document. In this target, set the appropriate condition, variables, stylesheet and media, page layout etc. So my target A has conditions A Only set to
Include, and B Only and C Only set to
Exclude (include overrides exclude). The values for the variables are set to the appropriate product name, release date, software version etc.
I've kept the description simple, but I also single source three WebHelp targets from the same TOC. This may not work for everyone, but if your WebHelp is based on the same content as your PDF, which, rightly or wrongly, mine is, It's easy to do. I have Print Only and Web Only conditions, different skins for each branding, more stylesheets for each web style etc.
Your second question. Should you put all your documents in one project? There are others here who can advise on that better than me, and the best answer for you will depend on the number of documents, writers etc. Flare certainly allows you to share topics and import topics between projects and to keep them in step (search the help for Global Project Linking). But you can also build multiple documents from the same project and people tell me Flare can cope with a single project with thousands of topics.
I originally planned a Global project, for genuinely global things, like stylesheets, page layouts, logos, web skins etc, a Shared project for content that is common to more than one output, for example, concept information that is required in more than one document. But in fact, most of my documents are currently in one project, in separate sub folders. I just have a separate TOC for each document, and a separate target for each branding of that document. The topics for that document are just in a subfolder of my Content folder. But as I port more content to Flare, as the day job and the release schedule allows, I think I may go back to plan A. I'm a sole author, so it's only me that is affected by my indecision.
If you do decide to use a single project, I'd advise you to make sure you have a clear structure and naming convention from the outset, as it's easy to get confused once you start serious single sourcing. For example, all my global content is prefixed with GLOBAL.
Keep posting the questions. Several very experienced and helpful MadCap users hang out here. I've learned lots from them.
