Short answer: in your case, there should be no problems.
If you need impedance matching (high speed signals) the size/form of pads can have an influence on the signal quality. But as long as you're not in the field of RF (radio frequency) stuff or interconnects in the gigabit per second range, you probably don't have to care about that with pads. For I2C, CAN (where impedance matching becomes important but still, pads are not an issue), I2S, SPI and stuff: no issue.
Components must self center in reflow
If the pads are not the correct size and form, SMD components do not self center anymore. But that's only an issue if you want to do reflow solder
Small components (ceramic capacitors, resistors) have a tendency to "stand up" on one of the pads which can be influenced by the shape of the pad. Also, not a big issue with hand soldering because you need to push down on the component while soldering anyway.
Access to Signals
Don't rely on pads to access the signals. Start using testpoints (put them in the schematic already). It will benefit you greatly in the future if you get used to having test points for every signal on your board - nicely labeled and documented.
Use LEDs as indicators on your board. Put indicator LEDs on every power rail. Put them on I/O lines (make sure that you don't exceed the load limitations of your components). Put them on TX/RX of your serial connections. Those LEDs don't have to be bright. Choose the resistor value so they just glow (way below 1mA per LED). It will make your life SO much easier when debugging stuff if you see an LED go out if a power rail is shorted. It's so nice to see if data is being transferred on RX/TX lines if you have communication issues. If you have an MCU also put a status LED on a GPIO line which you can blink with different patterns depending on what part of your software is running - it's a great help for debugging.