This is not say that you can’t use the ‘childish’ diminutives with adults, but contexts matters. The childish forms sound more affectionate, and are only really acceptable between partners and children and grandparents and grandchildren, and really only at home. In some cases, they are used among female friends and colleagues. Male friends usually won’t use them at all, especially not with another male friend. Even if you hear your workmate Tomáš’s mum call him Tomík, you probably shouldn’t.