Pimp, Start topping early then LST when she starts getting tall. (tall being between 1 and 2 ft)
mrT, I don't tend to run different strains because my grow is so small. The light pen from my 600w doesn't give me a lot of wiggle room for hight variances so I keep it simple. I have seen many people run multiple strains in the same scrog with great success, I have also seen people attempt to run multiple strains and fail horribly, its all about how much time you want to spend training. Get as much info you can on the strains before you decide to scrog them together, difference in harvest times and stretch can be the deal breaker. We know white widow can be branchy depending on phenotype, super lemon haze on the other hand can grow really tall really fast, a 6 inch difference in height from one plant to another can cause your lower plants to be sparse and fluffy while toasting the taller plants colas, that would be what I would take into consideration when growing multiple strains but I'm sure Legally has some more input as he is currently doing just that.
About raising/removing screens during growth;
Screens can be raised if needed in early veg, before the plant has really grown into it much. The screen starts acting as a support system once the branches have established in it, removing a screen that is holding up your plant's weight would be like kicking the crutches out from underneath a cripple, they need them to stand, buds don't like to grow upside down (tried it).