http://www.erowid.org/archive/rhodium/chemistry/pcp/imine_synth.html#Examplepce

It says a monoalkylamine could be used...  I assume that they use ethylamine (which has a ethyl group) and they make PCE, using Methylamine would make PCM. Could one use methylamine hydrochloride in this reaction? Free methylamine is kinda hard to work with, considering its a gas.

PCM is described as "Active at 15mg, only slightly less potent than PCP, but subtly different effects and worse taste and smell when smoked."

So, could methylamine be substitute for ethylamine to make PCM? Can any other pcp analogs be made via schiffs base?

Read up on the formation of imines.

The reaction equilibrium is ketone + amine base <---> water + imine. The KOH (or sodium sulfate) in your linked rxn is used to absorb water. You can use the hydrochloride, but at least some of the methylamine must be in freebase form. The rxn should be performed in an acidic medium, in order to protonate the carbonyl oxygen and catalyze the rxn; you will have some methylamine in freebase form and the other in salt form.