You can roast the poblano peppers a few days in advance if you like. Instead of grilling, you can roast over a gas burner. Grilling is easier but with roasting over a gas burner, you can clean off more of the outer skin. To roast over a burner: roast dry over the burner set to medium-high heat until completely charred, turning as needed. Afterwards, place in a plastic bag to sweat for 10 to 20 minutes. To clean: wipe skin off over a trash can or over the sink. After cleaning them, cut in halves and remove seeds.
You can use whole milk instead of 2%.
Pick a cooked, smooth, pureed salsa; the heat level depends on you and your crowd's preference.
I recommend sticking with poblanos, but it mixes well with large Anaheim peppers as well.