Aha! I have found much things out, being an avid hell cower.
1) You can go in Cow Portal at ANY level, lvl1 noobs etc.
2) You have to kill baal to be able to make in the difficulty(or diablo for classic), just sit in town while ppl kill baal when you are lvl 1 and u still get the quest.
3) Despite popular belief, if you can't make cows, killing king does NOT affect you until you are able to make cows, basically you can kill king many times, then level enuf to be able to do Imakecow games, you can still make cows.
4) If anyone kills cow king, and you made or can make, and you are,
A)Partied at the time
B) On the same screen, or even within 2//3 screens of the player
You will lose the ability to make cow king.
I haven't tested the theory of partying, and you are in some other act.
Thats about it.. and with some mh'ing frineds I have found out, cows are lvl 28!
That means they give better exp at lvls 28 or so, even if baal runs are WAY better. Hell Cows are lvl81, but baal is better till lvl 89 or so.. then you best relax for a bit, cause it takes 20+ runs after lvl 91 to get a lvl up...
NM cows, I haven't checked, because after Normal Baal runs, you are ready for NM baal runs, so I never have worried about it, they are in the 50-60 lvl thoug.