I just had to check which rules apply to some other words, like 'stadium' and 'forum'. Turns out that both 'stadiums' and 'stadia' are valid plurals of 'stadium' while 'forums' is the preferred plural of 'forum', although 'fora' is not incorrect.

[1]http://en.wiktionary.org/wiki/stadium#Noun [2]http://en.wiktionary.org/wiki/forum#Noun