do the dividers come out?

They need to be silicone in or something silicon then so these can ride in and out.

does it matter what order the baffles r in?

I would assume not, you really want to have your sump flowing in one direction and this helps that. I think common sense plays a big role here.

I would like to know the measurements of the tank that are not very obvious to me. There are more non-tank divisions

This kit is designed to be used with the standard 10 and 20 Gallon tanks. They should be measured with the Aqueon Tanks. 10 Gallon tanks measures "LxWxH" (20"x10"x12"). Other size options are for the 20 Gallon Tall/High (24"x12"x16") and the 20 Gallon Long (30"x12"x12").

I just received my kit for the 20 gallon long. Are there any instructions or guidelines?

No. Just follow the pictures. Wait! Can't really see them, huh? Getting a one-star review for lack of simple instructions. Nice clean cuts on the acrylic though. I bought one off of Ebay that had very clear and detailed instructions. It' on a piece of paper, and they just make copies.