You can definately OVER skim a tank. A tank that is over skimmed will have trouble growing certain types or corals. Some corals just won't open up fully in those conditions. If it's a really good deal, you might want to go ahead and buy it, but the solution to making it work on your tank would probably involve a timer, as in not running it all the time, rather than a smaller pump. The smaller pump may not drive that size skimmer. Most need the correct flow in order to get the bubbles into the chamber. If you run it with a timer, start by running it only at night. The skimmer will increase oxygen levels in the tank, which will help keep the ph from dropping as much at night. The ETSS is an expensive skimmer to start with, so you might just want to get a Berlin Turbo, which you can buy for under $200 including the pump. It is rated for 250 gallons! The Berlin is a proven brand. It will cost less to operate, since the pump will be much smaller than the other one necessary to drive a large skimmer like that.
Sonny