I think jimscott is onto something. Some sundews are bound to choke others out, but if you had some leggy species like D. capensis and then some little self-seeding/dividing ones like D. spatulata - or I was thinking that D. burmanii or the easy pygmy species could probably cooperate well. Byblis seems like another good candidate for such a planting since it's tall and has a scrambling habit, and since it sets seed and dies back regularly, it probably won't get so crowded that it will be a problem for the shorter species.
If you plan to keep the soil warm, take the species into consideration. I think D. adelae, among others, is a species that doesn't like warm temperatures - but I wouldn't know where I live. You also might have to watch out for soil fouling in an undrained container. To prevent the need to have to dig up all your plants and replace the soil mix, you can rig something clever when you plant it that lets you drain water out of the tank after it's settled to the bottom. Being able to drain the tank also means that you can rinse it out, say, if a loved one accidentally decided that your plants looked like they could use a heaping helping of Miracle Gro. If you browse around in the old threads you might come across some threads that describe using a reservoir at the bottom of the tank, like a very rough rock or gravel that can take up extra water, or an empty pipe or plastic storage box with some holes poked in to let in water and not media. I think Crissytal hold the record for the coolest implementation of that idea;
http://www.terraforums.com/forums/showthread.php?t=112574
In her case it serves a slightly different purpose, but the basic lesson is that it's always good to have a place where the water can go without the soil mix and plants' roots sitting in it. You might also look into the planted-potted type of terrarium design, where you hide pots in holes in the soil line of the terrarium, which gives similar advantages.
Peat moss and perlite is a good general-purpose mix. It's not critical, but if you can find a larger grade of perlite that doesn't have a lot of dust, I think that makes for a mix that breathes better and isn't quite as sludgy when wet. Sand can be used, but it can't be water soluble (like limestone sand) and I think that coarser grits are generally preferable. Watch out for brands of peat moss or perlite that come treated with time-release fertilizers.
Are you aware that some sundews take a long time to grow from seed? Easy, large species like D. capensis or adelae could be a bit of a wait, up to a year to the size you see in most pictures. D. burmanii is one commonly available sundew that matures quickly, as are many of the pygmies so far as I understand (I haven't grown any from seed, just gemmae, which are baby plants that bud off of healthy adults.) Drosera are quicker from seed than many carnivorous plants, as are their cousins in Byblis, but might it might take some patience on your part.
Best luck!
~Joe
