yeah, floating plants will block some light, depending on where they are relative to the light, and how many of them there are.
Some people corral them with airline and suction cups, or you could just try putting anything that floats in the tank (bits of wood, styrofoam, etc.), and seeing if there is a corner that floaty things tend to get pushed into. If the current does gather floating stuff into an area, you could see if having floaters there would work with your planned aquascape, or if you can monkey with the filter out put a bit to try and end up pushing floaters where you want them.
Anyways, if you do decide to go with floaters, you will need to periodically thin them out. I've got frogbit, and I really like it, and it's really easy to thin out when I decide to. I've heard good things about red-root floaters, but I haven't tried them myself.
I wouldn't worry too much about substrate feeding, especially if you have a fish that digs around or uproots things. You could mix in some gravel-like substrate with a high CEC (flourite, eco-complete, SafeTSorb, etc.), but don't think it will really matter with most of the plants suggested so far.