Seriously, the biggest problem that most people have with blogging (myself included!) is blogging regularly. From a traffic perspective, you'll want inbound links all going to the same site as that helps in general.
If you tag things well, that will solve 90% of your (described) concerns.
For successful blogs: Think of many news/opinion sites like HuffPo, TechCrunch, Forbes, and hundreds of others.
As long as your content is interesting and thought provoking, go for it. If you are just going to crank out half assed things, don't do that. Write meaningful content that is beneficial to your reader.
As reader, I return to single topic blogs more often than blogs which are allover.
So for now, I am got multiple blogs, but it is harder to manage.
Tim's been doing this -- and/or his own thing -- for years.
Some technical. Some photography. Some about "the kid" -- or more so, thoughts "the kid" raises.
Write about what interests you. Be careful to say something others will be interested in, and do so with some thought -- if you're interested in "growing" an audience.
Be authentic.
Thanks!