Should you put the pregnant platy in a separate tank from the other fish?
A platy's pregnancy lasts approximately 4 weeks from giving birth to giving birth again. This can be as little as three weeks if the water is warm, or up to six if it is cold. There is no point isolating her until she is less than a week from giving birth.
My personal opinion is that breeding nets and traps are unsuitable for livebearers other than guppies. Confining a fully grown fish to a space this small for any extended period (ie several days) is extremely stressful. It can cause an abortion of the fry or even the death of the mother. I would not recommend these devices except for raising fry in. I have used breeding traps but only if all of my brood tanks are in use and I see the first few fry come out of the mother. This way I know she will only be in one for a few hours.
The use of a separate breeding tank is very effective. I use a four gallon (16 litre) tank for this purpose. I use fake ambulia or cabomba, which can be packed away when not in use. These plants (and therefore their plastic copies) have fine fronds for the fry to hide from the mother who may eat them. For a period of only a few days I don't bother with filtration, I just use an airstone, but any longer will require a filter. An air driven box filter will suffice. As soon as the mother has finished giving birth I remove her. The fry can be reared in this setup or removed to another setup (or placed in a breeding box in the main tank.)
I am a serious breeder with a network of people who take fry I do not want or cannot look after - I keep only the best fry for my breeding program. A lot of serious breeders cull the fry but I prefer to give them away than kill them because they aren't pretty enough. If you consistently isolate your females to give birth you will find that you become overrun with fry. Having fry is fun. Having a large number of half-grown platies, too small to give away but large enough to need several tanks, is not. Do not save more fry than you can look after. Most pet shops will take fry that are pretty enough to sell, and many have customers who keep large predatory fish. These shops will also take ugly or deformed fry to use as feeders.