Here, I'd probably give you slightly different advice than your academic advisor. I think you should approach the optometry faculty as soon as you think you are interested in doing work for them. It's entirely possible that a faculty member is now thinking about his latest project in the fall, and wondering just who he will choose to work on it. After all, faculty members usually work year-round; research certainly doesn't stop in the summer.
Of course, you will want to go at this professionally. You should write up a resume or a current curriculum vitae, find out what area each of those faculty members works in, and do some reading in any areas that interest you. Then, when you approach those faculty members, it's entirely likely that there will be no job/money available during the summer. That's when you drop off the resume, and remind him/her that you will call again as fall approaches. However, since you are able to speak knowledgeably about the professor's research, your name will be prominently at the top of your professor's list as he/she tries to find research assistants later on in the year.
NSERC scholarships are great. You might want to check
them out for the optometry department. If you come
into their lab holding a substantial chunk of change
(ie. they don't have to pay you very much), your
chances of finding a job increase tremendously.
It is not mandatory at all schools to have a letter from a physician. However, you should check with the individual schools you're interested in. I do think it's definitely a big advantage to have a letter from a physician. After all, the admissions committee is trying to decide whether you have the qualities to be a good doctor. Who would be better to decide that than a physician who has already undergone all the training?