Is it possible to use 12 pt font size in the achemso package?

In a word, 'no'. The achemso class is intended purely to support submissions to the ACS, and so has no features which are not needed for that purpose. The achemso package is intended to support producing more general documents which need some of the features of the class. You should use a suitable general class (for example report) and load appropriate packages.

As of today, achemso's main font size is 12 pt. Quote from its source (https://github.com/josephwright/achemso/blob/master/achemso.dtx#L968):

\LoadClass[12pt,letter]{article}

Nevertheless, what you are asking for in general seems to be easily achievable. I wanted to use the achemso class (but not for ACS submission) and change the main font size (from 12 pt to 11 pt). This is what works well:

\let\titlefont\undefined
\makeatletter
\let\l@addto@macro\relax
\makeatother
\usepackage[fontsize=11pt]{scrextend}

This solution is a combination of https://tex.stackexchange.com/a/5341/11816 and https://tex.stackexchange.com/a/35967/11816 -- without the undefinition of \titlefont, there is a clash between KOMA titlefont and achemso titlefont.

I'm interested to hear about disadvantages with this approach.