[@Isotope]No, there's not a single issue with keeping BT-7 at its original stats.
My elves would basically have the equivalents of those but equipped with a 33mm autocannon. The story behind this is that the tank was originally supposed to double as airship defense but the crew couldn't see anything in the stuffed turret so that idea was scrapped. regardless the 33mm cannon got upgraded further and the tank basically became a nightmare to infantry, light vehicles and anything not reinforced to take grenades and small caliber cannons to the face.
They'e also crazy fast, faster than the original BT-2. Like I said before mithril is every engineer's dream.
Some years ago elves also realized everybody is armoring up their tanks so they introduced an upgraded variant to this with 99mm cannon (well, due to lower pressure tolerances elven cannons need to be bigger, count it as a 75mm) which works decently enough but regardless I have only light tanks with only enough armor to be not eaten by machineguns or with some luck tank a 33mm cannon shot at the right place.
My nation's current military doctrine is to advance really-really fast so the enemy won't be able to keep pace with us. Whether it truly works or not is still up in the air.