I think we are more of a 'must have a heater' country than 'must have a chiller'. Last year we had a few risky days where it got to 30 outside, but the tank water didn't get above 26 or 27, so it was close to where I thought I might see some dead bodies.
It sounds as though you have gone all out with your setup and I have always thought the madagascar lace plant is very attractive, and maybe you need Co2 with that sort of exotic plant setup, but generally I think shrimp are supposed to do better without it, but if it is all working stick with it.
I would love to see some photos, as I am sure others would as well.
No rush with the chiller, as we are going into winter. So shop around for the best price.
With such a small tank, get the smallest cheapest chiller you can find in the brands range.
Hailea chillers are probably the best compromise between price and quality. So you can start looking there. I think the HC100a is the smallest in this range.
The cheap eBay phase change chillers will work on your small tank but quality will reflect with the price.
Teco chillers are in the high range and will be a bit more expensive but they are good quality. I have a couple myself. And they have been running every summer for many years now.
The chiller is only useful if you live in a country that is hot, I think places like the UK should be alright without it. About the Co2, i'm not sure if it will affect neo's or the cardina, but my tank seems ok, I set the Co2 at its lowest because of a small tank, at least the plants are doing well, especially the floating one's mini water lettuce, duckweed, RRF and frogbits which is beneficial to shrimps, I also have Fissidens fontanus, and madagascar lace plants.
Forgot to mention, I also have a UV light.
If you want to see some photo, just let me know.
Glad we can be of some help and I agree you are doing the best thing by thinking it out thoroughly before you get started. It is very frustrating when you spend a lot of money on a small setup to then have to go and do the same again for another setup because it isn't big enough, way more economical and cost effective to get it right first time, though as you say you are starting with some of the stuff! I'm also sure that when I closed down the small one and moved the shrimps to the new tank that I probably lost half of the shrimplets as they were too small/young to be moved in reality but I really didn't want to have to run 2 tanks for a few months whilst I waited for them to grow - I had way too many anyway.
I can't be much help regarding the 'chiller' as we don't have hot enough weather here, in fact even the humans don't get that over here (except in expensive cars)? A lot may depend where you are placing the setup as well, mine is in a dark corner that doesn't get any direct sun. If it isn't likely to get too hot there for a while you can of course come back to that as a separate issue as it won't mean disrupting anything in the tank anyway should you decide to get one later!
You are probably best steering away from the CO2 as it is one less thing to set up and, as you, I have read a lot of reports saying it isn't good with caridina shrimp setups, though I have no actual experience with it, but why risk it.
Hope you will keep us informed/updated.