Wood Bleach and Washing Soda. Coin of the realm. Search for 99-06-02-012D, it's a GM service procedure for fixing the sludgy/coagulated coolant problem. The flush they call for is no longer widely available, but powdered wood bleach (available at some Ace hardware stores, and probably other places) and washing soda (I found Arm & Hammer brand at an IGA store) are the ingredients they call for. Some people prefer citric acid (from the bulk/health food store at 1 pound per gallon-apparently a Mercedes-Benz recommended procedure), but I think oxalic acid (wood bleach) is particularly good for the coolant-mix problem.
Doesn't hurt to hit it once with dishwasher soap (not dish washing soap, you understand, but non-sudsy soap that goes in the dishwasher) first to take out any oils and let the acid flush work. Flush well between soap and acid. It's a long, messy procedure, but it does the trick. The real key is getting it hot enough during the flush, and rinsing completely, with engine running, between flushes. Getting a good rinse is almost impossible without hooking it up to a hose (hot water is definitely better), so the Flush 'n' Fill kit really helps.
I think you're better off driving it around than doing the high-idle procedure in the bulletin, but it's up to you. FWIW, mine kept belching horrible crud for 3 iterations of this procedure, but it had mineral scale worse than I'd ever seen. All better now.
Once it's totally cleaned out, you can pick a coolant without worrying about interactions. Don't be shocked if it starts leaking after you do a thorough flush. I'm not saying it will, just that sometimes the flush dissolves some crud that was plugging up a little leak. I found a couple of o-rings leaking after the procedure.
Also, be careful with the acid solution, it will mess with the finish on metal and plastics. I wouldn't let it dry on any parts you care about the looks of.