I doubt if Taunton could cope with trains terminating from both Bristol and Exeter, so it may be better to consider an RE▸ service from Bristol to Paignton. This would give HST▸ connections at Newton Abbot, Exeter and Taunton and BRI» .
It could in theory. For trains from Exeter, they could terminate on the practically never used Platform 1 or the Bay Platform off of Platform 5. I'm pretty sure though that the Bay Platform isn't certified for passenger use. I have seen Track Maintanance Units sit on Platform 1 in the past.
Also services from Bristol can terminate on Platform 6.
Also Bear in mind that Platforms 2 & 3 are reversable and signalled at both ends
I have a feeling that Platform 5 is also reversable for trains from Bristol or Castle Cary. But don't quote me on that