If the details I've found online which suggest the 200EV is 3.4m high then I highly doubt smaller wheels would trim 30cm off the height.The Enviro 100 has smaller wheels; is it possible to fit smaller wheels to an Enviro 200 whilst still giving adequate ground clearance, and would it then be able to clear the low bridge?
Based on the fact they also need to keep some E200s for DCC tenders I suspect the only cost-effective way to convert the 90 route to electric would be to revise it as I've suggested in the Stagecoach Yorkshire thread. Having a handful of buses dedicated just to this route (rather than a wider pool including tenders) would be more difficult to manage. When the current electrics get replaced in due course a lower height bus is likely to be available, but that's not going to happen for a long time,