If using a glass tank use glass for baffles-if using an acrylic tank use acrylic baffles.
Allow a little room (about 1/8" on each side of baffles) between baffles and tank walls for expansion- Yes baffles can expand slightly.
Don't rush, take your time and let the silicon dry COMPLETELY before performing leak tests.
Remember once something is siliconed , if you need to redo it, you need to strip the first silicone out, new silicone and old silicone won't stick to each other well.