When mobilized, are all reservist brigades always infantry only?
Is it a good idea to keep only artillery and cavalry as standing army during peacetime, and use the mobilized brigades to fill the role of infantry when a war breaks out?
How should I keep my peacetime armies? Divided into groups of brigades garrisoned here and there or massive, unified armies? Should I keep them at the border or inside the territory?
There are lots of wars you don't need to moblize for.That means it's best to have infantry in your standing armies. Where to keep them: just make sure it is in provinces where they'll suffer no attrition (specially in the early phases of the game).
Adding to this, once I get sufficiently large, I do tend to keep a reservist support group (engineers, artillery, dragoons) in back in the middle of my nation with just a few infantry. My other armies remain well-balanced and do the bulk of the fighting (particularly in limited wars) so that I don't have to mobilize for small engagements, not to mention rebels. The reservist support group only comes up to the front when I mobilize, getting split into subunits and picking up reserve brigades on their way to reinforce my regulars.