Our local system.
As described, they are used indiscriminately. There are about two of the "old" types (90, 92 and 2K) for every one of the "new" types (07 and 12), and each service seems to have them deployed each day in the same proportion. Within each type the trains are also formed mixed of the various vintages. One thing you won't see is a train formed with old and new types together - I don't know if they can't, or just don't do it.
About 45 minutes at Canning Town, followed by the same at Canary Wharf, will pick off all in service. Nothing at Poplar that doesn't serve the other points, although it does overlook the smaller depot sidings. The larger Beckton depot is out of sight of any line, you need to hike round from Galleons Reach to even get a distant view, plus (from what I gather you want to do) have very good binoculars.
There's a detailed DLR working timetable in downloadable Excel spreadsheet format somewhere on the web that was made available in response to a Freedom of Information request, can't see it immediately, maybe someone else can point to it.