If the slot dimension is critical you might find it a little over size, if I needed a slot bang on 6mm I'd use a 5mm cutter.
The single flute carbide cutters are magnificent, I've done a lot of aluminium profiling with one using just air, with light cuts it keeps the cutter cool enough, I use a compressor and regulate the pressure to give the flow I need to clear the chips.
To direct the air onto the work I use a length of clip-together coolant tube with a canula needle epoxied into the end of the nozzle, I ground the end of the needle off square for safety.
I have a drip feed to introduce lubricant into the air flow which helps with aluminium and is essential with stainless ;-)
Vacuum will need a shoe over the work to lift chips out of a deep slot, if you had a 100% hard vacuum then you still only have atmospheric pressure working for you, about 14psi.

- Nick