Absolutely use an overflow box. I would never recommend anyone not use a box. I tried it and am not a fan of how risky it would be set up wrong, hence my comment not do it. You want the flexibility overflows bring.
I have never used the shadow but considered ordering the one from Synergy. I may still do that when I get around to that tank.
Unions thread together easily but keep things water tight. It makes plumbing in manageable chunks much easier and final assembly a couple minute job. It also allows you to take it apart after everything is solvent welded to repair, replace or upgrade plumbing, pumps etc... Great choice on the Reeflo. From a value perspective, unions are to plumbing, what a sump is to filtration, if that makes sense.