If you look at the level 2 on POT, you can see that there is a lot of potential for slippage, so its not suprising he encounters it when trading. It's even more true on the TSE: POT. The spreads on that are a few cents usually like 3 to 5 cents.
The only expensive "stock" (ie more then 75) i've found to date that is very scalable thus allowing you to increase your size to a few thousand shares and rarely encountering slippage or more then 1 cent spread is SPY.
Ideally i'd like to only trade SPY, but I got the PDT rule to get over first, and i definatly won't deposit the cash required until my results proven themselves.
But yeah i usually trade the same 3 to 5 stocks every day, mostly TCK, G, the big 5 canadian banks, and POT. I can get over the PDT rule by trading canadian stocks only.