To have a symbol add another symbol, the effect is "tiles_to_add". As an example, below is a recreation of the base-game Peach symbol:

effects = [
{
"effect_type": "self",
"comparisons": [{"a": "destroyed", "b": true}],
"tiles_to_add": [{"type":"seed"}]
}
]

To add multiple symbols in one effect, add another set of curly brackets with a comma in-between. For instance my modded symbol Pomegranate, which has the same effect as Peach but doubled, includes this line:

"tiles_to_add": [{"type":"seed"},{"type":"seed"}]

You can also replace "type" with "group" if you wanted to, say, add a random gem.

To display sprites in descriptions, insert without the quotes "<icon_yoursymbolgoeshere>"

Using base-game Peach as the example again, the description would be "Adds <icon_seed> when destroyed."

Groups are a little fancier because of the and/or. A recreation of the base-game Ore would look like this: "Adds <group_gem> or <last_gem> when destroyed."

Thank you for the help

I've found it helpful to look at the source for mods I've downloaded, you can find it in the subfolders of Steam\steamapps\workshop\content\1404850\

Also, if you want to add a symbol 10% of the time, you can do something like:
(this is based on how the Ocean mod does it)
{
"comparisons":
[
{
"a": { # Random number between 0 and 100
"starting_value": {
"rand_num": {
"min": 0,
"max": 100
}
}
},
"b": 10,
"less_than": true
}
],
"one_time": true,
"tiles_to_add":
[
{ # Add Suit
"group": "suit"
}
],
"anim": "circle",
}

Thanks, this will definitely help my modding adventures more. (No that isn't sarcsm)

What about making something happen after x turns?