Which mild pepper is a good substitute for Serrano pepper? I plan to make a guacamole dip and one of the ingredients is Serrano pepper, which is 10,000 to 23,000 heat units by the Scoville scale.  

That is way too spicy for me.  

Which one of the milder peppers (100-2,000) would be a good replacement?
Anaheim peppers make a nice substitute, first in reducing the heat (500-5000 scoville), but also in terms of its availability and similar flavor. It does frequently run on the hotter end of that spectrum though.