If you are going to go through the trouble of changing the plumbing you may as well move away from trying to fit Red Sea pieces in to how you want your layout. I am going through a similar process now. I had a leak in my plumbing so I started ordering pieces. Now that I have the pieces for the repair the leak has stopped so I am doing nothing until my ReefMat arrives. I need to cut the downpipe for the ReefMat so my repairs will happen at that time.
The easiest way to do this is buy 3 of the Red Sea R42221 pieces. You would take the hose barb off all 3 pieces. Well 2 if you are fine with the smaller barb depending on your return pup hose size. For myself I replaced the one that the fitting came with with a 3/4" barb. This is a standard items at Lowes. Sorry for the crappy pic but it is tucked behind my Plumbing Leak Fix-o-Matic (tm). I.E. a silicone cooking mat zip tied in place to funnel the water from the leak into the sump. This will let you run hose right up to the bottom of the tank and you have 1 piece of plumbing out of the way.
The next two items have a number of options to choose from. You can remove the hose barb that comes with the fitting and add a screw in fitting to convert from the 32mm Red Sea to any standard size - 3/4", 1", 1 1/2" etc. I plan to go to 1" because that was the cheapest gate valve I could pick up.
The other option is you can go with a union from CoralVue
Metric/Standard Union Coupling (32mm to 1")
SKU: PVC-UNCPLG-1
The union could be used in 1 of 2 ways.
Option 1: Cut your current downpipe and glue the 32mm union side to your downpipe. You would have standard 1" out the other side to use standard 1" plumbing from Lowes/Home Depot.
Option 2: Use ABS to PVC adhesive cement from Lowes to glue the union onto the outside of the R42221 fitting. This will allow you to move directly to 1" standard plumbing at this point. This option would also provide you the largest discharge opening out of the R42221 fitting.
I decided against the union even after buying it because I cant find clearance for it in my setup. If I go with option 2 the back wall of my stand is too close. I can't go with Option 1 because my current downpipe is leaking at the fitting so I can't use any of it. I also can't choose options 1 because both my downpipes have a 90* hard right just after the fitting. The run parallel to each other without enough clearance for the union.
1" gatevalve will replace the Red Sea valve whenever I make my repair. I'm going to go with standard white PVC from Lowes. You can order the same Gray color as the fittings if you wish. I am not concerned with it.
Like you stated, I also moved my ATO external. Once you do that you have even more plumbing routing options and you are not limited to Red Sea routing choices.
