For baffles it's best to use glass in glass, or acrylic in acrylic. That said, I have done many glass sumps with acrylic baffles. The concern with acrylic baffles in a glass tank is that the acrylic will actually absorb some water and swell, so they will crack a glass tank if fitted too tightly. The other issue is that silicone won't bond very well with acrylic, but for internal application like baffles or overflow boxes
IME the bond is plenty adequate as long as you use a good adhesive silicone (vs simple sealant grade silicone).
Again, IME you can never build an acrylic tank or other water holding vessel using silicone to hold it together, but for baffles or overflow boxes adhesive silicone is just fine.
As for the design of the sump, opinions will vary. If you want a refugim and a skimmer, and want to cram everything into a 20G it will be tight.
Skimmer selection, IME over the years I have come to believe that correctly sizing is better than oversizing. IME oversized skimmers often have trouble keeping a steady foam head and tend to not work as well as a smaller skimmer in terms of steady performance. You are talking about @ 60Gal of water so the volume is not that much so it can be difficult to find a good skimmer that small. Personally I have about 60 gals and I'm running a RLSS I6 and it seems to be well matched. There are endless other options, but that's what I'm happy with.
For the overflow box, Shoot me a PM. I can build whatever you want for cost of materials plus a 6 pack of good beer
