When I bought my guitar it came with an assorted size of picks. They're all the same shape and diameter but,the thickness is different on each one. When I first started practicing I would use a (1.0mm) pick. It felt the most natural in my hand. Now I realize that it's easy to strum with that size but, harder to hit single strings with it. To pick single strings I use the (.50mm) pick. Its flimsy nature makes is easier to hit single strings without touching the other strings. But, it's harder to strum with it.

Do professionals change pick sizes due to the songs they're gonna play or is this a bad habit that I need to kick early on?

Also is there any pick type and or thickness you recommend for a beginner?