No, no mess, sometimes when they build a bit of burr comb on bottom,
then you'll cut it off. This is about the right space for many reason's....like winterkill, bees cant get out and die inside of hive...lots of room to pile up on the board, also space for mite check/count with sticky board, I do mite treatment on this also.
No problem with waxed plastic foundation, they will use it.
Are you running two medium for brood? Then that will take 10 frames each, you need about another 20 frames for honey super. I run two deeps for brood, another two deep for honey.
Konrad

Thanks Konrad. I was thinking I'd need 3 mediums for brood and two (or three?) more for honey, because 3 mediums is about the same as two deep. So when I get my package should I start with one or two mediums? I was thinking i'd start with one and add more as it gets full.

This sounds good, only use as much as needed for space, they will go hard at work start building cells for queen to lay in. I remember when I started several years back, the queen laid first or second day already. Don't be surprised when the amount bees slightly drops in two weeks or so because you have no new bees for 21 day's at least and older bees will die off. Add another box when you see box getting filled with bees and about 2 frames not drawn yet.

I use an all-medium system. Nice and modular.