So, quick tip that I found very helpful - LiveAquaria has quick stats on fish final size and minimum gallon sizes for each fish. These aren't perfect but, while you're learning, are a very good rule of thumb*. Another good resource is igreens nano fish guide, which gives a list of fish suitable for tanks to to 30g IIRC.
If you want to go really advanced with planning a stock list, in addition to asking here, there's someone named eatbreakfast on humble fish who is EXTREMELY knowledgeable about fish, and they'll give you feedback on if your stock plan will work.
EDIT: Now with links.
Igreen's nano fish guide:
https://www.nano-reef.com/forums/topic/74703-lgreens-ultimate-guide-to-nano-fish/
EatBreakfast's humblefish thread:
https://humble.fish/community/index...ish-plan-work-post-here-im-here-to-help.5122/
*: One of the exceptions to this are very active fish like tangs. A general rule - which is not agreed upon by everybody, but one that I think is good to follow - is that smaller tangs (bristletooths/etc) require at least a 4' long tank given how active they are, and larger/more active tangs (acanthurus like achilles, etc) require a
minimum of 6 feet
.