The first part of this statement is not correct.
Only work experience
in Canada while being a full-time student in Canada doesn't count. Work experience
In a foreign country while being a full-time student there counts. I can confirm this since I got PR and almost all my work experience was from a part time job during my (full-time) undergrad.
It is correct however that unpaid work experience never counts under any circumstances and it is irrelevant for your application what kind of duties you had during internships/co-ops etc.
About the original question: The only safe bet is ONE letter that states all of the necessary items
- Your title(s)/position(s)
- Your duties
- Details about period of employment, number of hours etc
- Your salary plus bonuses
Everything else (like a paystub plus a letter about your duties etc.) might work but it is a gamble. Believe me, it is worth putting a lot of effort into getting a crisp and clean letter from your employer. Your work experience usually is the only "soft criterium" where the CIC officer uses his/her own judgement (Education and money and health and language etc. are quite straightforward). Don't give them the chance to decline your application b/c of insufficient proof of work experience.
That said, better to have some other documentation than no documentation.
