The original question was could you use them for a week or two. While the safest idea is definitely to remove all lead weights before you plant, I don't know if a 7-14 days' exposure is enough to cause damage to your fish. As I understand it, lead (and other heavy metal) poisonings ususally occur from long-term exposure to the metal. However, best to be safe.
Since you are discussing lead weights, I assume you are referring to stem plants, which usually are held together with these weights. (If not, please disregard the rest of this message.) I take off the weights before planting because it's usually what's best for the plant, regardless of the fish.
When you remove a lead weight, you frequently find broken stems and rotting leaves. These problems do not help your plant to root and become healthy. In addition, most plants like to have water circulating through them, and a lead weight restricts water flow where you'd like it most -- namely, from the place where the plant should be rooting. If the stems of some plants are too close -- myrio is a good example of this -- then the plant will begin to rot at the base, whether the problem is caused by a lead weight or just by planting the stems too closely.
To get a good planting, try these steps:
1) make sure you have enough of a substrate to hold your plants down. I'm no expert on sand, so I don't know what depth of sand you'd need. For gravel, you probably want about 3-4" of gravel. This gives you the ability to plant relatively deeply and also gives your plant the opportunity to develop a good root system. (It also traps more "junk" which will become a natural fertilizer for the plants.) If you are working with 1" of gravel, consider adding more.
2) (with stem plants) remove the lead weight, rinse your plants well and pick off any snails you may find. If you'd like, you can give them a soak in potassium permanganate (Jungle's "Clear Water") for 10 minutes to kill bacteria, then re-rinse.
3) with a sharp knife (again, just with stem plants), cut the base of each stem off. If the stem looks healthy, cut about 1/4" from the bottom. If the base is crushed or if it looks like it's rotting, cut up from that.
4) carefully, with your fingers, strip the leaves from the bottom inch or so of the stem (measuring from the newly-cut section). You want to pull the leaves off as close to the stem as possible without damaging the stem itself.
5) plant the "stripped" part of the stem in the gravel, being careful to keep the foliage above the gravel. The deeper you can plant, the more difficult it will be for your fish to uproot the plant.
Cutting the base of the stem and stripping leaves will send a signal to the plant that will cause it to devote its energy to producing a new root system. The places where the leaves were stripped are the nodes of the plant; these are the same areas from which the roots will emerged. Leaving foliage in the water (and not under the gravel) will reduce the possibilities of the plant beginning to rot.
In essence, your best way to keep your plants from floating is not to keep the lead weights on, but to develop a strong and healthy root system as quickly as possible. If you have large or active fish, you may find yourself re-planting some stems over again, but in my tank stem plants tend to hold their own after about a week or so. So try to be patient with the fish if they bounce against them. If you do need to replant, you don't have to re-cut the stems, just push them back into the gravel.
Also, to keep water circulation good, try to plant only 2-3 stems in one spot. You can plant 3 stems together and another 3 stems 1/2" away, to create a fuller look, but try to avoid the temptation to grab ten stems and push them into the same "hole". You will probably end up with rotting plants later.
Hope this helped,
Pamela