Hello korweabo0,
Interesting setup you are creating. After reading your recent post I see why you are connecting them. If I were you I would draw the 15/30 gal tank the same height as the 20 gal tanks for clarity. Without the dimensions I was wondering how you would pull this off.
Water weighs 8.35 lbs/gal so:
20+20+15=55 gals * 8.35 = 459 lbs + gravel, decorations, etc...
and
20+20+30=70 gals * 8.35 = 584.5 lbs + gravel, decorations, etc...
I don't know how much help I can be on the stand but if I was building one for this setup I would definitely go with at least 2x4's for legs and 1/2" - 3/4" piece of wood on top depending on what you are using (plywood or pine or ???). You wnat to brace the legs at the top and bottom for best results. Someone else may help better with the stand design.
I have never connected fish tanks together with pvc but have connected 55 gal barrels and 275 gal water totes together for pond filtration using 3" and 4" electrical connections like shown here in post #2
Birdman's Fluid Bed Filter - Koiphen.com
The electrical connectors screw all the way together unlike standard pvc fittings and I used this to seal them
PL Sealants & Adhesives
This product is black in color so clean with mineral spirits and use gloves for easy cleanup.
If you want them a certain color you can paint them with Krylon Fusion spray paint which I believe is fish safe.
Are your tanks glass or acrylic?
I recently made a pvc overflow and a sump and it works great. I used a 2" PVC standpipe with slots at the top no lower than the level of water that would fit in my sump. The 1" pvc pipe then went inside near the bottom of this. Had a little trouble with noise at first but added a ball valve before the sump and cut it back a little at a time until the overflow into the pipe got quiet. Working great for the last month or so.
I don't have any diagrams now but aspects will post some good ones I am sure.
Remember to calculate how much water is in each inch of all your tanks to determine the size of the sump and the depth of the overflow inlet so during a power outage you don't have a flood.
Hope this helps.