Hi..
Almost certain that a taxi licence wouldn't be enough... since presumably you will be transporting passengers on a 'hire and reward' basis, the minimum requirement would, I think be a Spanish D1 PCV licence. Taxi licences are extremely difficult to obtain anyway and normally very expensive.
I'm not certain but I don't think your UK (EU) licence, even if it has class D1 minibus cover up to 15pax, can be used legally for vocational purposes in a Spanish registered vehicle.
Special rules will apply for insurance cover and also the vehicles will be subject to annual ITV tests irrespective of their age.
You need to talk to a lawyer specialising in vehicle law, and to the Trafficos.
fb