If I go with a cartridge filter (Pentair Clean and Clear) and I get the one with the 1/2 HP pump, can I put any size filter in there?
No, the housing is sized for a specific filter size.

The PNCC0150OF2260 is a 150 sq-ft filter with a 1 1/2 HP two speed pump. The pump is a little large but if you are up for the task, you can swap the impeller of that pump with the smaller version. Impeller kits are usually less than $50. However, having the low speed of the 1 1/2 HP will save quite a bit as is. If you find you need full speed for something it is nice to have. You can always decide down the line to swap the impeller.

As for opening and closing a pool with a cartridge, it can be done effectively but you just need to take a few more precautions. Closing the pool after the water gets below 45 degrees and opening after is rises above 45 degrees will help minimize the amount of algae that is needed to be cleaned out. In fact with my own pool during the winter, I don't close the pool and my SWG shuts off for a couple of months because the water temp gets below 50 degrees. However, I don't bother adding chlorine because the algae does not seem to grow at all at these water temps. But cleaning up a lot of algae from a pool with a cartridge filter is not a lot of fun either.


A sand filter requires fairly high flow rates for backwashing so you just need to make sure the pump can provide enough flow rate for the filter. In most cases, this should not be a problem and the combo units have pumps that are more than large enough for the task. For your size pool, I would normally suggest a minimum 2.3 sq-ft (250 lbs) filter to minimize the backwashing frequency and to minimize head loss. You could get away with a 100 lbs filter but it just won't be as energy efficient and you will need to backwash more often. Again, combo or not, I would go with a two speed pump so you can run on low speed most of the time and use high speed just for backwashing and vacuuming.

There is truely no "right" choice for your decision only trade offs. You have to decide what is going to work best for your own situation.