400 SFM is fine for HSS in most aluminum alloys and I'd suspect that breakage was due to excessive run-out and/or too heavy a cut. If you have run-out issues, achieving a "good" surface finish will be difficult no matter the end mill used.
I watched the video and I don't know the type of machine you're using but it looked as though Datron was using a machining center, with a MQL setup for 'coolant'.
What machine are you working with?
You will want to keep axial depths high and radial depths low to stay productive, but it is all dependent on your approach to toolpathing, If you cannot generate low engagement angle toolpaths, your axial and radial depths are totally dependent on the amount of hp available at the tool.
A 2-flute 6mm will probably run fine at .002 inch/tooth as a starting parameter in 6061-T6.
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