I'm confused here, wouldn't tiomáint be changed to "tiomáineoidh" since the original form is two syllables and should thus follow the second conjugation rules?
It has more to do with the ending than the syllables. Also, it's not tiomáint, but tiomáin, and it's used as first conjugation because of the áin. Sadly, there are a few you'll have to learn.
I see, so is there a good way to tell when this happens? I saw that úsáid was given the same treatment when conjugated, does it have to do with how the ending syllable is stressed?
The third-person future conjugation is itself the best way to distinguish between first conjugation verbs and second conjugation verbs. If it ends with -fidh or -faidh, then it’s first conjugation; if it ends with -eoidh or -óidh, then it’s second conjugation. The syllable count method to distinguish between the two types is a good guideline, but it’s not completely accurate.