Be aware that acrylic/plexiglass is inherently easy to crack if overtightening metal fittings. The smallest crack will continue to propagate once it happens and eventually start to leak once it gets big enough. I've personally lost two reservoirs to the problem and it's why I only use plastic fittings instead of metal if they screw directly into the acrylic. People can and do use plexiglass, but you need to realize how fragile the stuff is before you start using it or you will be having expensive replacements. 
 
If you're going clear then I recommend skipping the coolant and using just distilled water + algaecide. It will act as a full biocide so nothing will grow in the loop, and there's nothing to break down because all coolants with dyes do eventually break down sooner or later.
 
I use soft tubing but I'm lazy and I prefer the flexibility, there's nothing pretty about how crammed full of stuff my case currently is. I will let others speak for hard tubing as I don't have personal experience with it, but it does have its own advantages/disadvantages. You will want a lot of extra tubing if teaching yourself how to do your own bends. 
 
As for fittings, they are probably "silver-plated" and you don't want those, once the silver goes you will be using whatever metal is underneath it. I recommend against using silver coils as well, no need to add extra metals to the loop or have that stuff running through the pump. Nickel-plated fittings are fine in copper loops, most blocks are nickel-plated already. Brass works too, though brass has zinc in it so copper will discolor eventually, but it's just cosmetic. Don't use aluminum anything unless the loop is 100% aluminum.