Does RO/DI water needs to be cycled before adding fish?

Well, RO water vs cycled aquarium is 2 different thing you RO water is clean from other toxic tap water chemical but absolutely NOT cycled. Your new setup tank don't have any Bacteria to deal with bio load from fishes yet.

still needs cycling and you may need to add removed minerals if you go that route?

Move all the filter media, substrate and decor over from old tank and you should be good to go. Since these have established BB already.

Since you're RO changes the waters TDS, you need to acclaimate your fish by introducing RO water into old tank through smaller w/c (20% at a time)

justarn said:

still needs cycling and you may need to add removed minerals if you go that route?

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this, I think you will need to add trace elements back in... you will also be making a ton of waste water, I hope you have some ideas on using it elsewhere

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kevincao said:

I will still try to put the old water in if possible. (maybe delaying the pickup date for the old tank)

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TDS (total dissolved solids) will be extremely low from RO water and if you put fish from a very high TDS straight into a very low TDS, it could be death. You don't hear a lot about TDS but it can be just as bad as pH shock.

Messing with water chemistry is a very risky practice. Greatly increase chance of things going wrong. Its less of a headache/heartache to have fish that can adapt or tolerate to your water than to alter it. GL

You should be fine with a Ph of 7.8. I keep south american fish in a Ph of 8.3 and have no issues whats so ever. I thought about using a RO unit but figured it would be too much of a hassle to add back minerals etc.. If I were you Id forget about using the RO unit unless you understand water chemistry very well.
As far as adding fish immediately, If you have filter media from your old tank that you can transfer to new tank that would help alot. If not than try Tetra safe start. I've used it several times and have had great results and was able to add some fish right away. Are you planing to add all the fish you mentioned at one time?

Since every answer in here is "I think" responses I'll tell you how rodi and freshwater works.

You need to settle and you need to air ate and you absolutely must re-mineralize.

You need to settle and air ate because rodi comes out at 5.0ph. Just by settling with an air stone it will bring the PH back up.

You need to add minerals back to the water using either Seachem Equilibrium or Replenish.

Using RODI is not altering and messing with your water. It's more consistent then relying on city water to always contain the same exact mineral content. My tap ppm varies 150 or so + just morning to night sometime and I only know because I have it monitored. Using RODI an absolutely clean water to begin with then adding back exactly which elements I want in the water and in what concentration is much more reliable and consistent.

Is it much more involved? Yes! Does is seem like it's over complicated to most? Sure! There's a reason why the best reefs utilize RODI as a water base because it's that extra constancy that makes the extra difference.

In the reef world as little as 5-10ppm swings can become harmful to the most delicate of species.

noside said:

Using RODI is not altering and messing with your water. It's more consistent then relying on city water to always contain the same exact mineral content.

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I never stated nor imply that RODI changed water chemistry. It was a general statement since it seems that's what the OP was set out to do.

kevincao said:

I decided to purchase a RO unit to reduce the PH because the water here is really bad. Thanks.

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I also think its a bad idea to just throw fish from city tap water straight into RO water