score:1
First, a little bit of nitpicking with your itinerary. Your flights will (most probably) be:
1a Montreal > Asian city (Beijing, Guangzhou, Hong Kong)
1b Asian city - Chiang Mai
2 Chiang Mai - Bangkok
3 Bangkok <> Krabi or Phuket (to go to Phi Phi Islands)
4 Bangkok > Singapore
5 Singapore <> Kuala Lumpur
6a Singapore > somewhere in Asia, Europe, Middle-East, USA
6b somewhere > Montreal
Now about your question [which isn't really about visas]. If you buy only (1) and (6) you could, technically, be denied entry in Chiang Mai, as you wouldn't have, yet, a ticket leaving Thailand. However, unless you look really suspicious, you will probably be let in. I have been 100+ times to Thailand, and have never been asked for my onward ticked there.
Once you're in Chiang Mai, you're good to go. Chiang Mai-Bangkok and Bangkok<>Phuket/Krabi are domestic flights. Bangkok-Singapore and Singapore<>KL are fine, since you have (6), a flight leaving Singapore.
If you want to make absolutely sure, you can buy in advance (4), Bangkok>Singapore. With that you have everything you need to pass through immigration checks.