It sounds like you want to have a custom shower system with both a hand shower and a showerhead. This is very do-able. Typically I would recommend you stick with the same brand, mainly because then you know the finish will match perfectly. Especially with finishes in the oil rubbed bronze category, you will find great differences between various manufacturers and it might look strange if the finish is not exactly the same. In the case of chrome finish, you may find there is almost no difference between companies. In that case there is no reason you couldn't have a hand shower made by one company, and a showerhead made by the other. You still need to do some other planning as to how your shower system will be configured. Perhaps this article we wrote about the topic will be of help. Delta Shower Systems Example of a custom shower system with hand shower and showerhead

Just go buy something at Lowe's or Depot. They have the some darn plastic stuff.

The flow will have to do with the actual shower head, not the rail. I have sold both, and personally like the way that the Kohler slide bar bracket slides up and down, better than grohe's.

There's an even cheaper option 3: Leave the rain shower and the handheld outlet in place. Mount a second shower wand holder at the height you would want a regular shower head to be. That can be done for the cost of the shower wand mount, $40, and a $5 tile drill bit. If your wand has poor flow, replace it with a higher flow wand head. More seriously though, why get rid of the rain shower head? Why not leave it in place for the occassional use by someone who likes a rain shower? But install a new wand that runs on a sliding bar. Open the walll from the closet side and install any necessary bracing, so your sliding bar can be screwed into wall studs or cross braces. That allows it to double as a grab bar, a useful safety feature for older people.