So I can stake any amount of coins, right? Would someone staking 7,500 coins receive half the reward of a masternode owner who had 15,000, or is there a specific set of advantages/bonuses which apply only to the magic number of 15,000?
Technically yes but in reality no as there are other factors. So while block rewards are split 45% Staking, 45% Masternodes and 10% treasury so MN and Staking are equal in that respect it comes down to the weight of each side of the network. If there is more XSN in Masternodes then Staking would be more profitable however at the minute there is more XSN in Staking so Masternodes are a bit more profitable. Obviously once Masternodes start running additional devices then they get access to additional income streams from service fees as well.