If I set my session to 24 bit or something, does ardour use 32 bit internal processing or only when I set my session to 32 bit floating point.
Ardour’s internal processing format is always 32 bit floating point (which is equivalent to 24 bit integer but without the risk of truncation). The format on disk has no relationship with the internal processing format.
BTW: several other DAWs nowadays have switched to 64 bit FP for the internal processing. Have you ever considered doing the same?
It has been considered, though the benefit from doing so only really comes in when dealing with hundreds to thousands of connections IIRC. Pretty sure there is a couple of discussions on here somewhere about this.