Yes, you can use it to cycle the tank. You could even use the filter in your own tank. It could instantly cycle your tank because it's actually the filter youre trying to cycle when we talking about 'cycling' as only alittle good bacteria settles in the substrate.
Are you going to use both tanks? Or are you moving the guppys over?
If you plan on using both, when you have an ammonia source to feed the bacteria growth, look inside the established filter and if theres 2 sponges, rince 1 of them into your new tank or filter or even place the sponge inside your new filter. Make sure you have an ammonia source present before you do this. If you opt for the rincing technique only rince one of them otherwise you could mess with the cycle of your established guppy tank, then after 2 weeks rince the other one. By this time you can bet your tank will be ready for fish.
If you are struggling to find bacteria food, you can let fish food rott in the bottom of the tank and this will create ammonia. Even better than that, youre from the UK like me so can get something called, SeAquariums - biomature. This is an ammonia based filter starter that ive used to cycle 3 tanks. Works well!
As for gauging the size of that tank without seeing it in the flesh.. id say 70litres max. What are the dimensions? It wiukd be easy to work it out if you had the length width and height.