As his joining wasn't "typical" of most Trill, Tochi endured something of a traumatic experience when the symbiont was implanted resulting in him being very aware that he's often thinking thoughts attributable to past hosts... in those times, he's more prone to using the plural pronouns. When he's alone with his thoughts and when those thoughts are largely his own (i.e. not directly identifiable as having stemmed from the experiences/memories of any of those past hosts) he uses singular pronouns... If you read back through some of the earlier posts for him, you'll sort of pick up on the pattern.
Again, this fact isn't commonly known but, I'm sure, there's mention of it somewhere in his service and/or medical records for those who have the clearance to access such things... and it may have even been discussed among those aboard the
Peregrine who know Tochi well enough for him to be comfortable discussing the topic (or where it's almost a "need to discuss" issue)... Drake, for example, is well aware of Tochi's past hosts... Vaela is aware of, at least Dirven (as Dirven had a fling with her grandmother and Vaela put two and two together)... Thorson knows as much as he can handle as he and Tochi were friends in the academy (although, in those days, Tochi was unjoined and his surname was recorded as Tigan) and often drink together.
Being a Trill is complicated.