Here's how the MIDI packs work:
They translate MIDI signals within it's established parameters and use that to generate dimming/switching commands on each channel of the MIDI pack.
Using a fixture such as the one you mentioned(Cosmic) is not a clear answer. In a nutshell, YES, you can, but you have to either have the channel full ON or full OFF. Anything less and the lack of power can damage components. If the MIDI pack supports switching, you're good. If not, then make sure you go full ON or off for that channel and you'll be fine. Since you're going to be programming stuff, you should be fine.
My suggestions are as follows:
You might want to look into a controller that will do what you want that also has a MIDI interface, and the MIDI implementation will do what you want as well. Might take some digging.
Also, you're going to probably want to invest in a smart dimmer pack such a the DP-DMX20L, if I got the name right. The latest versions are switchable per channel for either dimming or switching. ParCans use dimming, while others use switching(off/on). Depending on what you get, you might want to think about DMX fixtures as well as what you can do with MIDI via the controller.
Or, you may consider products such as MyDMX and Compu Live. Be aware that the prices differences do equate to feature differences, and in my personal experience, MIDI seems a bit unreliable in MyDMX right now. However, in my scenario, I currently don't have any need for MIDI and lighting to cross paths.
Not knowing more about the MIDI packs, this is what I am saying. Of course, you can buy more MIDI packs, But, personally, I'd never do a single MIDI cable run that exceeded more than 30 feet from first device out to last device in the chain in, as even short cables show delay after 3 units in a line. I just wouldn't feel happy or safe running long MIDI cable runs as it was never intended for that application. The fact that it CAN be done does not mean it really SHOULD be done. Then again, HOSA makes a 50-foot MIDI cable, so despite wisdom saying "hey, keep it short", there is a market for these long wires.