| Pea Gravel is the way to go. Rinse it off first. pennywort is okay but if you are going to have other plants in there they are a hog and you have to constantly cut them back which makes them grow even faster! I got rid of all of mine in the small pond. Just too much work too control their rooting system (because ya cant *lol*). If you do put clay soil in the bog filter do not put more than an inch on the bottom and cover it all up with pea gravel assuming it is a fairly decent size. Anything more than this will make your water cloudy and dirty looking. You will have to start all over and you dont really want to do that. Well maybe you do but I hated it! *S* I suggest leaving out the soil completely because any plant you have in there will get their agrofom tabs and all the yummy stuff filtering up through it to feed your plants. (and this is THE only dirt I suggest using for any aquatic plant. Rule of thumb is the nastier the clay soil is the better your plants will thrive.) The cardinal flower is a beautiful plant and well worth any replanting you might have to do though I do not suggest it in a bog of this size. When you plant in a smaller pond like you are doing, you want to keep your plants in it simple and plant ones that wont overrun your filter with a enormous root system. Irises are wonderful, as are aquatic mints (We grow chocolate mint in one of ours and wow it is nice smelling and very pretty). Just remember in a bog filter this size to keep the plants simple. Pickerel weed will look nice but I personally think it belongs in the pond itself or in a larger area. It just gets way too big for the bog filter of this size. Hmm but in reading your post again you never did say how big the bog filter -would- be....can you tell us? Here is a website that can maybe help you out:
http://www.fishpondinfo.com/marginal.htm then you can look around for pictures of the plants. | 
