Species profiles go by footprint rather than gallons because what's most important is room to move. I would not have anything except snails in a 5.5. I also would not suggest shrimp; just not enough room for shrimp to live comfortably with a predator.
Most site recommend a minimum of six fish if one gets a shoaling variety like Rasbora or Tetra. However, that changes when you put them in a tank with a predator. I asked Rachel O'Leary (msjinkzd on YouTube and www.msjinkzd.com
) one of this country's most knowledgeable "fish people." She said no fewer than 10-12 if one is forcing them to live with a predator as I do
So, I recommend a tank that is no smaller than 30 x 12 x 12 (20-gallon long) for all residents' needs to be met. If you only have one shoal, a well-planted 20 x 10 x 12 (10 gallon) will work with 10 or so smaller Nano fish like Chili Rasbora or Ember Tetra.
I admit that, in the past, I have not followed the above....much to my regret because once I did I found shoaling species housed properly act more naturally and live longer than when crammed in a tank too small or with too few members.
And tell your Mom you're sorry; but no Pleco in anything less than a 30 x 12 x 12 tank. They just poop too much for anything smaller. Of course, this bolsters your argument for a 20 long so go for it.
I can't comment on Platys or Mollies because, TBH, I don't like them and they create an incredible amount of waste. Also, I want my Betta to be the centerpiece of my community tanks. When one adds equal-size species, in my eye, it takes away from the Betta. Species profiles indicate a minimum tank size for Mollies is 36 x 12 x 12 or 30 gallon; for Platys it's 24 x 12 x 12 or 15 gallon. I could find no profile which suggested a 10 gallon was acceptable for either.
Guppies are fine, though. Don't pay attention to those who say you can't house Guppies with male Betta for whatever reason. If a Betta can't be housed with Guppies he (or she) can't be house with any other fish or shrimp.
FWIW, I find a 20 long the most versatile. You can do so much both with planting and dividing and choice and number of shoals.
Almost forgot: You can also have African Dwarf Frogs but, from experience, I would only house them with Plakat. The long fins of a HM look too much like food to these near-sighted critters. ADF do well in bare bottom tanks which makes cleaning so much easier.
And there you have it: TMI. I do hope it helps you somewhat.