And does a device that runs at alternate current only, mean that it will only run at a set number of cycles/frequency?
Yes. A universal motor has the armature connected in series with the stator windings (unlike a DC motor, which usually uses a permanent magnet for the stator). Reversing the power supply reverses the fields in *both* windings; and so, whatever phase they were in (attracting or repelling) before the current changed direction, they will continue to be in afterwards. To change the direction, the current in just the armature must be reversed. All other types of AC motor run at a speed which depends on the supply frequency, since they rely on the reversals of the supply to change from attracting to repelling and back. Transformers are designed to work over a particular range of frequencies; too high and eddy current losses come into play, too low and magnetic saturation rears its ugly head. Capacitive-dropped power supplies (found in some low-power devices where isolation is not required) are also frequency-sensitive.
Universal motors will run on both AC and DC, but the performance will not be exactly the same. With DC, the motor will be able to deliver rated torque at a little higher speed. With AC, the motor speed will drop more when the load is increased. A device that runs on AC only may run properly only at the designated frequency and voltage, may run well at either 50 or 60 Hz, or may run well only if the voltage is increased or decreased by the same percentage that the frequency is increased or decreased. It all depends on the device type and design. If it is a device with an AC motor (not a universal motor), the speed will increase or decrease by the same percentage that the frequency is increased or decreased. The mechanical consequences of the speed change may be more important than the electrical consequences of the frequency change.
Only universal motor can run on both AC and DC. If you want to modify an AC motor to run on DC, then at first you need to know constructional details and working principle of DC motor. You can read the working of a DC motor at the link given in the source.
