The pools profitability has nothing to do with its size, but more with what you get awarded from and what the fees are.
Adding 50 TH/s to any pool should have no effect on its profitability.
Keep in mind that on a small pool you might wait longer to get your payout.
If you are looking at renting I suggest maybe looking here:
https://www.miningrigrentals.com/Thank you. Is duration a factor? I mean will I get better returns doing 50 TH/s for a day or 600 TH/s for 2 hours (theoretically the same number of total hashes)?
miningrigrentals.com is one of the sites that I have been looking at.
--Jacob