With what you already have, I'd suggest using the 70-250 with a quality close up lens like the Canon 500D. The other option is to use extension tubes with your zoom, but that can be awkward.
I wouldn't recommend using the 50 with either extension or close up lenses because you'd have to get so darn close to you subject. Also, because it's a 50mm lens, you won't have the background control you'd have with the longer lenses (narrower angle of view with the longer lens).
Here are some ways of getting close: http://www.barbeephoto.com/articles/close%20up/get_close.htm
Probably the best reference on this subject is John Shaw's Close Ups in Nature.
And yes, your focusing plate is probably the easiest way to focus. No need to leave focus at infinity, just use it to fine tune.