There's various types of great stuff out now, some that supposedly expand even more than the original but I haven't noticed much of a difference. There's yellow, red, black, and blue that i've seen at our stores. Remember you want GreatStuff not DAP, the DAP is actually a soft foam, you want the hard carve-able foam. GreatStuff does not adhere to smooth surfaces like glass so you'll want wood or cork or something for it to grip on.
As for your silicone issue, definitely do painters tape. It gives you nice clean straight lines without worrying about gouging your plexi with a razor. But if you do get any on it, let it dry then just rub it off. Sometimes if it's really thin and got smeared when it was still wet you'll need alcohol on a paper towel to rub it off.
Hope that helps

And happy vivarium building!