Fellipe:
You need a separate container for the working strength fixer. For example, if you have a 1 liter container for your working strength film fixer, you would put 200ml of concentrate in that container and then add 800 ml of water, for a total volume of 1 liter of diluted (1 +4) working film fixer. You would then pour the needed amount out of that container into your film tank, and when fixing is complete, pour the used working strength fixer back into the same container.
As far as how much you can re-use the working strength fixer, Ilford gives capacity information on their Rapid fixer pdf:
http://www.ilfordphoto.com/Webfiles/2006130218312091.pdf
As I read that document, it indicates that 1 liter of 1 + 4 working strength fixer has the capacity for 24 rolls of film. I would use that as a very rough guide, and recommend a clip test every 4 rolls of film or so.
I keep track of my fixer use by putting clear removable ("Scotch") tape on my bottles, and making a single mark for each roll developed.
For the Ilfostop, you have it right.
Matt