Hi there!
Contrary to popular belief, the bigger you go, the better. That's because maintaining a tank means keeping a balanced ecosystem, and that's harder when you have less water to work with. So with that said, the most recommended minimum size to start with is 20g - Big enough to have some variety and be manageable for maintaining water quality, but small enough to fit in a tight space. But if you can fit bigger, definitely do. It's easier, not harder.
An important thing that some beginners don't know is you need to prepare your ecosystem before adding fish. This is called cycle. There is a sticky here on the forum with some
beginner articles and you should read over each one.
Now to your fish list:
Tiny catfish - Do you mean coryadoras? They are my favorite. So cute. They are very social though and need to be in a school of at least 5. There is two tiny varieties, the Pygmy or Dwarf Cory, that stay so small that you could keep a school even in a smaller tank.
Pleco - It would need to be a small variety if in a small tank. Many plecos actually get to be over a foot and need large tank! Look up the pleco BEFORE you buy and take note of the maximum size and be sure it will fit.
Betta - These actually can do fairly well in a community tank, but don't put them with other "finny" fish like guppies that they might see as competitors
Shrimp - I can't remember if bettas will eat shrimp or not. Someone else will have to chime in. They make for a good clean up crew.
Snails - good clean up crew
"eel shaped fish" - You probably mean something like a Kuhli/Coolie Loach. They are fun, but another group fish, so you want to get 5-6+. These are also fish that stay around the bottom of the tank, so if your tank is small it would be crowded with both kuhlis and cories.
A good way to check if you have picked good fish for your tank is to use
AQ Advisor. Make your "wish list" and decide your tank size. Then enter your tank size at AQ Advisor and add the fish to your list; it will then suggest if your tank is overstocked. It's not perfect but it is a great guideline when you are trying to figure out what and how many fish you can fit in your tank. It's also a reasonable resource to warn you about potential compatibility problems.
Remember - ALWAYS do your homework (or ask here) about any fish you are considering. A schooling fish that is alone will be a sad, stressed, unhealthy fish. A fish that will grow way too large for your tank will be a sad, stressed, unhealthy fish. google will be your best resource as you prepare!
And again - ALWAYS CYCLE YOUR TANK FIRST! It takes a while but that is time where you can research, plan your stocking, and read up about the hobby. And it's worth it in the end.