I'm starting to plan to build a bike and am looking at parts. I found a frame I like (Poloandbike CMNDR frameset) which is chromoly, but am now trying to find a seatpost to go on it. I believe the seatpost on the frame is 31.6 mm. I found a seatpost I like (Kalloy Laprade SP243) which is made of Al6061T6. My question is, is it alright to have an aluminum seatpost on a chromoly frame? I know some alloys will need grease or different conditions, so I wasn't sure if this would be an alright things to do. Any help would be greatly appreciated.
There isn't a problem with putting an aluminum seatpost in a steel frame. It's a pretty common thing to do.
Make sure to grease the seatpost before putting it in, lest it get stuck. If it does get stuck, you could try ammonia or drano (sodium hydroxide) or chilling the thing (e.g. with a CO2 cartridge to unstick it). See this page for other stuck seatpost suggestions.
Also, make sure you have the right diameter seatpost. This can be done by checking the documentation for the frame, or using a seatpost measuring tool (which most bike shops should have), or a pair of vernier calipers.