It is a simple request, yet the implementation may not be equally simple.

The problem is that the spells SummonI-II are technically the same spells for ARC, SCH and SMN.

So they would have to add new spells or create a separate window/menu to configure your pets appearance for each spell.

Both methods would need coding and testing.

I don't think it is high enough on their prio list right now, but I would welcome the possibility to choose the pets appearance in the future.