I know lots of people use 15.5 Keggles for brewing 5/10 gallon batches without a problem. I figure it is useful to go for the 20 gallon pot so when I eventually do a 10 gallon batch I wont have to worry about boil over. My question, will I will run into any problems doing 5 gallon batches in a 20 gallon pot? I also plan on using a 15 gallon cooler mash tun and a 20 gallon HLT. Does this sound like a good setup?
The only issue with doing small batches in a large kettle is the liquid may not come up higher enough to reach the thermo. You can see in our sig how we have a kettle with horizontal couplers to get over that problem.
Do you guys recommend the cooler mash tun or should I just get a third pot and use it for a mash tun with a false bottom? If I use a pot what is the best method for maintaining temperatures or conducting a step mash?
Unless you have a way to heat it (herms, rims, or direct heat to the pot) I'd use a "cooler" style mash tun. I had a cooler for a few years and it worked fine until I designed my herms system, which now has a pot with a false bottom for the mash tun.
Unless you're just using existing kettles, there wouldn't be a need for a bigger HLT than the MLT. For a 10 gal batch you would never need more than a 10 gal kettle as a HLT. The big surface area in a big kettle will impact your evaporation rate, but you can account for that.