Mark - Current Tiagra (4600) will work with up to 30t. Different brands of chain are okay - for the 6/7/8 speed chains, the dimensions are the same. I've got a Shimano chain on my eight speed Campagnolo-equipped bike.

You might want to talk to your LBS and see if they have any older RDs that would fit. The big cog would be the only hangup, and that can be accommodated to an extent by adjustment. Also, lower-level RDs would work - I've got a Shimano Altus RD on my commuter that works well, and that bike came with a 28t big cog.