If you’re not going to process the film yourself then just keep rolls of various ISO on hand and use film according to light conditions. If you were going to process yourself you could push or pull processing according to an exposure you had worked out with a particular ISO. It’s true that ISO 64 has finer grain than ISO 400, that said if the smaller particles are getting less light more of them are going to be washed away during processing leaving you with a small grained but grainy none the less print. As far as color balance-accuracy, is that what you really want? Back in the day I mostly used slide film for its color saturation. I loved to use Kodachrome and print on Cibachrome, red objects in particular looked awesome on that shiny stuff. If you’re just sending them out with no instructions you’re going to get what they give you which can vary wildly. It sounds like you have a huge backlog of images so it may be wise to invest in a decent film scanner. Once you digitize the stuff, it doesn’t matter what film you used providing the negative-slide isn’t badly underexposed. You can saturate, color balance, and sharpen to your hearts content in Photoshop.

If the end result your after is digital, when you’re done scanning your 5000 slides, sell the scanner and buy a nice digital camera and cut out the middleman.