There is no RFC for community strings. But the implementation could vary from vendor to vendor. For example, Cat5K family do community string indexing using the @ character, see:(ftp://ftp.cisco.com/pub/mibs/supportlists/wsc5000/wsc5000-communityIndexing.html). IOS 12.x and later can handle community string lenght upto 231 characters whereas CatOS can handle upto 21 characters.
But you also need to make sure that the NMS application does not have any limitation when it comes to special characters for doing the polling. In general, you can look at RFC 1157 for SNMPv1 and RFC 1902 - 1907 for SNMPv2 related details.
