Hi Sinus,
I can only speak to US FDA regulations here.
Yes, you can use non-standardized symbols IF they are accompanied by adjacent text.
See: 21 CFR 801.15(c)(1)(i)(C)
In that case you do not necessarily need to include it in a symbols glossary since a glossary is only relevant if you use symbols without adjacent text. If you however do not have any adjacent text you are not allowed to use any non-SDO (standards development organization) symbol. Mostly I would go by ISO-15223 to determine if anything in there might be applicable for you.
If you checkout FDA Use of Symbols in Labeling: Frequently Asked Questions (Question 10) they specifically mention the use of "homegrown" symbols.
Can I use "homegrown" symbols, i.e., a symbol that is not established in a standard developed by a Standard Development Organization (SDO), without adjacent text?
-> No.