(I've moved the topic here for now, since I understand you're currently using the Free Edition.)
Yes -- what you describe sounds fine to me, assuming that your iMac meets our prerequisites:
https://www.hauptwerk.com/learn-more/requirements/
https://www.hauptwerk.com/clientuploads/documentation/PDF/HauptwerkPrerequisites.pdf
https://www.hauptwerk.com/clientuploads/documentation/PDF/HauptwerkBackgroundTechnicalInfoOnComputerHardware.pdf
... which I expect it would (unless it's quite old).
k2bhm wrote:My plan- connect direct to my iMac, 2 alesis vi61 keyboards. Connect up by usb and Midi boutique reeds witches and box of tricks
If you're connecting everything via USB then for maximum reliability it might be best to keep all of the USB devices connected to the computer via a good-quality powered USB hub, since that should ensure that all devices have sufficient power and should hopefully guard against the operating system renaming/renumbering them (and thus having to auto-detect settings for them again).
k2bhm wrote:Do I need a midi/audio interface?
Not necessarily. The built-in audio output on Macs gives excellent performance (polyphony/latency/etc.) and has pretty good audio quality. For best audio quality you could optionally always add a dedicated audio interface later if you felt the need.