It depends on the size of the animal. O. scyllarus grows slowly, so you don't have to worry too much about it outgrowing its tank. Like I said, a 20 gal system is fine for a 3-4 inch animal if it is well run in. For a large 5 inch + O. s, I would like to see the animals in 40 gal or more. Acrylic is not essential if the glass is 7 mm or more, but I would recommend putting some acrylic strips around the bottom edge. You don't need an entire sheet, just a 3 inch or so strip that lies on the bottom. The corners are particularly vulnerable. I want to stress, however, that this is necessary only for the largest O.s. in a 5 - 7 mm glass tank. Even then 99 times out of 100, the animal is not going to break or chip the glass, but if you want to be safe, a little acrylic where the animal might try to dig is advisable.
My other concern was skimming. O.s. have a habit of taking more food than they can eat and then burying it. This can rot and cause problems. A good skimmer can help in these situation, but they cannot compensate for a well run in tank. If you don't over feed, have a well established tank, etc., you can probably get by without a skimmer, but again, it provides some insurance.
Roy