In imperatives, sometimes we see in Ca'NoN {-'eghmoH}, and sometimes
we see {-choH}. For example: {yItam'eghmoH} or {yItamchoH}.

I don't know, if there is a Ca'NoN example of using just the verb,
e.g. {yItam}, but regardless whether there is indeed such a case or
not, I wonder..

Should anyone choose whatever he likes ? Is there something of the
three (-'eghmoH, - choH, or none at all), one should necessarily use
in an imperative ?

We see all forms. I don't think Okrand made up his mind about -'egh + -moH until KGT, so some earlier words violate it. I also think 'egh + -moH may not apply to every imperative on every be-verb; it's used for things you have to actively do yourself, even if that's only a grammatical pretext. I don't think -choH necessarily negates the need for -'egh and -moH.

-- 
SuStel
http://trimboli.name