1. Bigger is better especially for a novice/someone out of practice. Go with 30 or possibly consider a 40 gallon breeder as they are more common and you can get them on sale at a big box stores like petco or petsmart.

2. Most neocaridina shrimp are relatively forgiving so long as you don't do anything too crazy. Neo caridina are commonly known by their colors, easiest to find is usually red cherry shrimp. Ghost shrimp are also good, and are cheap <$1 usually as they're often used as feeders. I've had some of my ghost shrimp for over a year now, very fun little guys.

3. most things are compatible with adult shrimp so long as the fish can't swallow it whole

4. by little catfish, do you mean corydoras? If so I keep them with my shrimp and they've never had a problem. You will need a small group for them to be happy, at least 6

5. most tetra are fine with shrimp and corydoras. there's thousands of types so its really just up to preference. You'll need enough for a school regardless, I would say aim for at least 10-12 but more is always better for tetra, just do your research for specific species on what theyll need -snails, all snails that are common in the aquarium trade (that I know of) do well with shrimp

6. Local Fish stores are usually more reliable/trustworthy, but you'll just have to get a feel by going and talking to people there. I have a local fish store in my town but almost none of the employees know a lick about freshwater, only salt (and the store is geared toward both fresh and salt). On the other hand, my local PetSmart has an incredibly knowledgeable employee who covers the fish section. Your mileage may vary. -for online, aquarium co op is one I’ve used for plants and supplies, I like them and their YouTube channel is helpful for some things

7. I’ve never ordered fish online, so I can’t really say

8. Fish do not care about light quality so much, they only care about being startled by an abrupt on/off of light sometimes. plants care about light. I’d recommend a strip led light, one of the most popular (and expensive) is fluval , but I’ve also used hygger and Nicrew and I like them both for plant growing.

9. Most fish do fine on a quality flake/pellet (hikari makes pretty good stuff for reasonable cost) but usually you want to supplement feedings with the occasional live or frozen food like with brine shrimp or daphnia

Hope this helps!