ISP Weld On #16 works great for a chemical bond. It won't fill big gaps so the cuts have to be smooth and straight. You should have that covered with the laser.
1/4" is thick enough for cutting threads into. I cut the threads with the tap and plexi under water to keep it cool. You can use vegatable oil as a lubricant too but it's messy to clean up.
I have used both brass and plastic fittings in plexiglas. I just use silicone sealer on the threads of either fitting now and just tighten the fittings finger tight. If you use thread tape, the sharp plexi threads can cut the teflon tape as you screw it in. Go too tight with the brass fittings and it can crack the plexi.
Depending on the size of your res and system flow, you could have a problem with turbulence even with staggering the fittings. Flow too close to the top can hit the side wall and churn up bubbles that get recycled through the system. A small baffle could help break up the flow.