I’m about to buy a new IPhone 4 and i want to install a spy software that i haven't decided yet but my question is this...

Once is jailbroken and Cydia installed will be able to download whatever spy software i get?
And once installed will it add an APP to the phone?
If I erase Cydia app so the person don't know about it will this clear the spy software?

Please correct me if I’m wrong... I’m totally new to spy-software a friend at work recomended...