Any capacity battery can be used with any motor John, only the voltage is important in that respect. It's rather like any size petrol tank can be added to a car.
The speed control is fixed by the controller up to the motor's maximum revs at the supplied voltage. If your motor is already at it's maximum revs in that small wheel 'tryke, (I suspect that may be the the case, especially if it's the 20" wheel model) that's it. If not at maximum revs, a controller may have an adjustment available, but for advice on that you'd need to contact either Powabyke or the manufacturer of your controller model. Just possibly, a search online might reveal some individual who's successfully experimented with those controllers to up the speed to the motor maximum.