But how did the Cylons find the algae planet, random luck?
That really wasn't answered in the episode. I think someone (Gaeta) was mentioning "fate" being that the Colonials and Cylons met at one of the sacred places with the system's star about to go nova on them. But I would hope there would be a more fitting answer than coincidence... the galaxy is quite large for that though I've had certain instances of meeting someone I knew thousands of miles from where both of us live. Anyway, I degress.
A plausible answer to the Cylons arriving for the algae planet would be the same reason as the Colonials. Food. Baltar needs something to eat.
Another thing that I can see is more planet-stops along the way. Not just for food and water, but to find more clues for the path to Earth. They got the Arrow at Kobol, now the Eye at the Algae planet...