If you are adding it to a glass tank, silicon would be the better choice but you will need to apply it liberally since silicon doesn't stick to acrylics very well. I've heard that black silicon works best but I have no direct experience with it.
Weldon cement is best for plexi to plexi. #3 or #4 wicks into the seams and bonds the plexi permanently. #16 or higher can be used to fill gaps. Silicon is still the best choice for plexi to glass.
Absolutely! Weldon cement is commonly used in making acrylic sumps, aquariums and and other items in this hobby. I built a sump and several overflows that have been in use on my SW setup for almost a year now.
Unlike silicon, Weldon and similar products actually bond acrylic pieces together. The seams can be virtually invisible if done properly and are extremely strong once they are fully cured.
Yes. Not as clean or straight as a circular saw, router or (preferably) a table saw with a good blade, but it should work fine for cutting a tank divider. Use a new, quality bit with narrow kerf and high tooth count and make sure the plexi is fully supported near the cut at all times and you should be fine.
I'm gonna be making a paludarium for my mudksippers and want to stick the rocks/gravel to the plexiglass divider and I'm gonna have a sort of ramp leading into the water from the land area which I also want to stick some small gravel too.
Any suggestions?
That's a tough one... If the rocks and gravel are not too heavy, silicon might hold well enough for that. Coating the acrylic with a thin layer of something else (portland cement or Krylon paint maybe?) might provide a better surface for the silicone to adhere to.